Just carried out this job as a DIYer, and it was pretty much as straight-forward as a boiler repair could be. This does not involve any gassy components, but folk should still be aware that they need to take care if there are not to accidentally knock any other parts during this task. This is a guide for those who wish to DIY this job, but they need to make the judgement that they are up for the task and take full & sole responsibility for the outcome.
Tools needed: Torx T20 driver (note this needs to be of a suitable length - check), silicone grease, pans and towels...
1) Turn both DHW and CH panel controls to 'off', turn panel power switch to 'off' and ELECTRICALLY ISOLATE THE BOILER AT THE NEARBY SUPPLY POINT.
2) Remove the boiler's decorative front cover. This is secured by two Torx T20 screws, one in each bottom corner just underneath the cover. These may also have 'slots' in them so a flat-blade screwdriver could also do this job. Some Torx screws have a security feature of a small 'pin' in the middle of the Torx hole so that only hollow Torx drivers will fit. If so, you have a choice; use a hollow Torx driver, or else use a small flat-blade screwdriver (eg a leccy 'terminal' driver) to prise and snap off that wee pin - they snap easily, and now any Torx T20 will fit.
3) I'd recommend removing both side panels to allow both hands to get behind the boiler to help guiding out and replacing the P2P, so remove the two (I think...) Torx screws down the front of each side panel, and also the single Torx screw which can be found along the top edge of each panel. Once removed, the panels can be swung open sideways, but are best fully removed by pushing them backwards to disengage them from the main boiler and allow them to be removed (hold them upwards during this too as they'll want to drop down).
4) Use a large flat-blade screwdriver or suitable spanner/wrench to turn the CH Flow and Return isolating valve to 'off' - ie turn through 90o. These isolating valves are on the outside 22mm pipes. Also isolate the mains supply pipe - this is 15mm.
5) Turn on a hot tap to remove any pressure from the DHW circuit - it'll likely dribble for a second only - and close it again.
6) Fold forward the front control panel to give access to the gubbins behind - see photo.
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This is how: Place towels under the boiler - just in case. Place suitable trays on the towels there - I used a large roasting pan. Ok, slowly undo each of the 2 screws one turn at a time, pressing - tapping - the screws in each turn to break the seal. Stop as soon as water begins to escape! Use these screws to control the water's escape. I filled a medium-sized sauce pan with the water - don't forget that the main exchanger is full and this needs to drain too.
Carry on until no more water comes out - it will drip for a while, of course, but it should become obvious when it's 'empty'. Undo the screws more fully to check this is the case. NB ensure no water comes forward and lands on the control panel. This shouldn't happen, but just keep an eye to be sure to be sure.
9) Right, you should now be able to fully remove these two screws, and that's all that's holding the P2P in place! Push the P2P backwards to disengage it from the boiler, and reach round from both sides - but especially from the left - to remove the exchanger and extract it from the LH side of the boiler.
10) The P2P is sealed using 4 rubber O rings, one over each hole. These might be stuck to the exchanger or left on the boiler. Reach in behind the boiler and remove them if necessary - they sit nicely in recesses. Use a damp cloth over a finger to wipe clean these recesses afterwards.
11) You can now either clean the existing P2P using strong chemicals such as drain cleaner (have a look on YouTube for this) or else replace it with a new or reconditioned part. Make sure that any replacement comes with new O rings at least. I managed to buy a new and unused item on t'Bay for only £30...
12) OK, I'd strongly suggest at this point you check the expansion vessel pressure as this needs doing when the water side is depressurised as it is now. Use a tyre gauge for this, and it should typically measure around 0.75bar. I was astonished to find mine was completely at zero, even after having some major work carried out on the boiler a year or so ago. Strangely, this didn't cause any problems and my system pressure - hot and cold - was always stable. Go figure. If water comes out the 'tyre' valve, this usually suggest your EV is shot. If needed, use a bike or car pump to bring it up to around 0.75bar.
Also worth undoing the blue cap showing underneath the boiler panel, above where the P2P was sitting - this is the condensate trap and could be full of grit and debris if it hasn't been cleaned regularly during services. Clean it out thoroughly and wipe a good smear of sili grease on the inside before replacing - keeps it nice and watertight.
12) To refit the P2P first thoroughly clean the O rings if you are reusing them (best to replace, tho'...) and then coat them liberally with silicone grease. Reach round from the sides and insert them fully in to the recesses in the boiler components. I'd also add a blob of copper-ease or similar (even sili grease will do) in to the P2P screw fixing posts to ensure easy removal 'next time'... Manoeuvre the P2P in from the LH side until it's aligned and push it forwards until the two screw posts engage - this happens easily. (If, for any reason, you find this tricky or fumble around for a bit or are stupid-clumsy, it might be worth checking you haven't dislodged an O ring in the process...). Refit the 2 screws and do them up evenly and firmly, but don't over-tighten.
13) Time to re-pressurise. I did this by first opening all the isolating valves I'd previously shut - the CH F&R and the mains - and slowly opening a hot tap, running it until it stops spluttering. Then turn on the boiler mains power supply and finally the boiler front panel power switch (leaving the CH and DHW controls still off). Expect some hissing from the boiler as the auto-air vent operates. The boiler will likely not do much and the pressure gauge will show 'zero' or near-as, so now use the blue top-up tap to refill the boiler. As soon as around 0.6 bar is reached, the boiler will likely start up and start circulating the water around it, and more 'hissing' will ensue. Give it time to settle and stop again, and top-up as required until you get it to around 1 to 1.2 bar, which is usually an ideal level.
14) Once the boiler has calmed down and stopped farting out air, you can then turn on the CH and DHW controls. Once the boiler has stopped priming - as it'll likely do - turn on a hot tap to check all is working as it should.
15) After checking that there's no leaks, refit the side panels - push them tight against the back wall, engage the curved 'hinge' with the back frame of the boiler casing, and then pull them forwards to engage - swing them back in to place and refit the screws.
16) You may find that every time you top-up the pressure using the blue tap, a dribble comes out a component (the 'by-pass' I believe) at the front left-hand bottom of the boiler. This is quite common - it happened to me. Chances are it'll settle and not cause any further issues, but it is a bit disconcerting when it happens.
17) replace front cover, pat yourself on the back, have a nip.