Scrape off the loose, feather it in by sanding, prime it, fill it and start again.

Is there many spots affected?

From photo #1 I would have guessed paint on wet paint but given photo #2, it is the blue paint that is lifting and it could be lifting even with the filler on top of it, so in photo #1 the thick cracking is most likely the blue paint and with filler on top of it.

My main concern is that even if I scrape down all the peeling paint that I can find, and get two coats of the top coat to stick, will the finish last? Or will I have a peeling / cracking wall in 6 months time?

What paints have you used by the way?

You said you are certain you washed the paste off ... the way to tell (too late now) if you got paste or not on the wall, when the wall is wet with water if your finger tips can slide on the wall too easily then there is still paste on the surface. It usually needs a lot of clean warm water and scraping and scotchbrite to remove the paste.

I've noticed that the cracking / peeling paint is only happening over the polyfilla - whereas the paint is sticking well to the blue coat of paint. Unfortunately there's a lot of polyfilla...

Also, in general the paint on polyfilla seems pretty weak - even if there isn't any cracking, if I stick a piece of masking tape over the polyfilla'ed area the paint comes right off quite easily (whereas for other areas of the wall which haven't been filled, no paint comes off with the masking tape).

Is it possible that I've got a bad box of polyfilla? As I mentioned, the peeling / cracking only happens where I've painted over polyfilla, and when I scrape off the paint the paint peels off the Polyfilla leaving the Pollyfilla behind in the wall. I've done a misting coat on the polyfilla every time, 30% water 70% emulsion.

I ordered the Polyfilla off Amazon (powdered version), and while the polyfilla had plenty of good reviews, I've since heard lots of bad stories of ordering DIY supplies off amazon (third party suppliers buying nearly-expired / generally bad products and selling on amazon for cheap). There's no expiry date on the box, does the powdered stuff have a shelf life? It's mixed well and seems to stick to the wall well enough.

Answering in random order.

Most/all Fillers are gypsum + other things based. Plaster is gypsum based and does have an expiry date on teh bags. Fillers do not have an expiry date but I am certain that Toupret for some of its products says 18 months from when leaving the factory ... so if a shop rotates its stuff you should not in theory have a problem.

I have used some own brand fillers (Crown Trade's own brand back then...) you did not need to sand it ... you rubbed it with your finger tips and it came off. If your filler washed off whilst whipping and not washing down the wall to remove the dust it is suspect.

An older filler will remain soft and crumbly and will probably be a lot more porous than it should be. Perhaps try pressing with your finger nail where there is filler and where there is no filler and see if you leave the same indentation on the wall. A possibility in your case is the liquid got absorbed too quickly in filled areas leaving the solids of the paint on the surface ready to lift off when they got wet [but the paint should have bubbled instead of cracking]. The good thing is you can easily put it right with elbow grease, sand or scrape or just get a sponge with water and wash it off and then fill again :-(

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If you decide to buy toupret from toolstation it is easy enough to make it up to £10 (e.g. buy a couple of rubber tubs for mixing the filler and something else) and get free delivery which needed £10 spending.

You got the paint now so that is OK. But the general rule of thumb here is do not buy from the DIY shops, buy from independents or buy from the companies own trade shops you might have one of these near you if you want to try next time:
in alphabetic order
Crown Trade often cr@p accuracy
Dulux Trade
Johnstone's Trade
Prices to you and me are probably a touch cheaper than F&B but not by much.

What's the difference between Toupret and Easi-fill? I haven't used either, but seen the names come up quite often.

Should I keep looking for paint which pulls off when masking tape is removed (paint which hasn't cracked or peeled)? or am I being too aggressive with removing paint?