It could be a faulty check valve yes.
Thank you. Not being a plumber, I have no conception of how likely it is that such a valve is faulty.
As for the other people offering 'advice' to me: I am a lawyer by profession. I often offer assistance on legal forums. If someone has a problem with a second-hand car, they don't find it very helpful if my advice is 'buy another car'. They want to know what their rights are as against the car dealer.
I've looked up the Water Regulations, and as far as I can see
"It is not illegal to install a product such as water fittings in the UK without the WRAS mark but you may have to prove compliance in other ways. It is legal to design, manufacture, import or trade in non-approved items. It is legal to use non-approved fittings."
In this case, water from the cold water system - which is potable - is possibly finding its way into the hot supply, which may not be. (In the house which my ex-wife and I built in the 1990s, in which - amongst other things, she did the electrics and I did all the plumbing and heating - the hot water was potable.) That's not a health risk.
I'm not in breach of 3(1), 3(2) of the Regulations, section 6 doesn't apply to me, and the tap isn't listed in the Table in section 5(1), so I haven't committed an offence.
I have to fix the problem, and if I must, I'll get an approved tap. But if I can comply with the Regulations by replacing a £5 defective valve, I'd rather do that than spend £100 on a new tap. I don't think that's an unreasonable attitude to take. Work on this flat is costing me a bloody fortune as it is!