Why dont you just do what we do when we "plaster" over on a damp proofing job? I have even renderd a whole swimming pool this way & no leaks!
1) Knock of plaster
2) SB2 the walls, 3 brushed coats, 90 deggrees to each other
3) Waterproof render coat 3 & 1 (plastering sand/low salt)
4) Skim up (not in a swimming pool obv, lol)
5) Masonary paint as a primer to emulsion.

No damp would come through this way, even if wall was holding back a pond, honestly!
EDIT
Think I will find the proof, so you don't think it's BS. I single handily built a swimming pool for a disabled little boy under his house a couple of years ago, to help with his physiotherapy needs .It was then I discovered the versatility of SB2.

Thanks for the pointers Chris. To improve the installation of the bay window I think I'm going to go for the thermal boarding with vapour control. The cost isn't that different, plus I should be easy to finish.

Dot and dab... and a bit of a tidy up.

How will you dot & dab if using a vapour barrier I am asking myself? but good luck with your project pal

Dot and dab will hopefully leave a bit of a cavity. I've gone for a thermal boarding from knuaf in the end. I'll upload progress pictures in due course. Taking down the plaster has been the biggest job so far. Cheers Chris

Aquapanel thermal is designed for floors but it will take a skim if it needs to although the manufacturers might not agree. The main reason for not using it is the price.

Thats what I use in bathrooms/shower rooms Joe, £12.99 for a 3 x 4. Yep that would be a good idea too.

SB2/SBR
http://www.feb.uk.com/products/bonding- ... febond-sbr

Just found this topic, so sorry it's a bit late.

No-one seems to have asked you where the moisture is coming from? All that has been proposed will tackle the damp symptoms not the cause; if indeed the moisture is rising from the outside ground then you will still have cold damp walls behind your new boards.

Have you checked the roof of your bay window? (They are notorious for giving trouble with leaks.) Have you looked at how the gutter above the bay window performs when it rains heavily? Is the room heated? Is it ventilated? Are the other walls cold or damp? You could always hire a dehumidifier for a couple of weeks and see if it improves matters.