Flex, and especially for signal/control like this, is normally considered perfectly OK for fixed wiring, so long as the slightly higher mechanical damage risk is addressed - so it may need a bit of trunking if it goes up the wall or along the skirting surface wired or something rather than clip direct- though in new build I rather hope it has conduit or capping behind the plaster.

I'd not blink - the regs certainly permit it. The 0.75mm may be a bit thin if that is the only CPC to something that needs one, depends on the fuse or breaker, but that is normal rules, and the thinner floppier stuff is probably a better match to the sort of terminals on small 'stats. - avoids  the 'tail wagging the dog' problem.

Mike

Hi Mike

Thanks for your reply. Yes most will be either in conduit in a plastered wall or concealed above a plasterboard ceiling so mechanical damage won’t be much of a concern. I just wanted to hear other people’s opinions before I carried on but I think I’ll go with the flex, should even make connecting into the heating manifolds a bit easier too

thanks again

Peter

Hi Mike

Thanks for your reply. Yes most will be either in conduit in a plastered wall or concealed above a plasterboard ceiling so mechanical damage won’t be much of a concern. I just wanted to hear other people’s opinions before I carried on but I think I’ll go with the flex, should even make connecting into the heating manifolds a bit easier too

thanks again

Peter