No hot water in one tap

We have hot and cold water in all our mixer taps except one, in the bathroom one works the other is only cold. Could the problem be the tap? It is 36 months old, and this has just started happening. Whichever way we open the tap, we get only cold water. If we open the bidet tap, it runs hot, then we turn it off, and there is a short burst of hot water through the sink tap – which then runs cold!We have checked for cal build up – none, tightened up braided hoses, which worked 3 times to fire boiler up, then stopped working again. All taps and shower are Roca.