As always Puss, it could be one of a number of things.... gas valve/burner/thermostat............. most likely, the Hot Water heat exchanger is partially bunged.
Nothing you can do I'm afraid. Time to call someone in. If he can unbung it, it shouldn't cost a lot.
He has just done a hot water temperature censor thingy, makes no difference.
our central heating is fine. oh dear.
He is now having another read up about the problem before hopefully getting it sorted after the weekend. many thanks for your help Builder x