I take it from what you say that the boiler doesn't kick in if a hot tap is turned on. If that's the case, I guess the most likely cause might be a failed flow switch in the hot water taps' supply line from the boiler.
I suppose so. If it is the valve at fault, changing it would hopefully fix the problem, unless there's something else wrong as well. If it still doesn't work after that, it eliminates the valve as the sole cause. I'd then look towards the flow sensor. Then I guess you're into the realm of an electrical problem either in the signal from the flow sensor or in the signal to the 3 way valve for hot water (or something in between). I'm getting out of my depth now so I'll "shut up". Hope you manage to resolve the issue. As a DIYer who has never experienced a combi, I'm sure there are people on here with more knowledge than I have who may be able to help if you need any further assistance.