Thanks for all your assistance JohnD - much appreciated.
Got a GasSafe engineer out yesterday who diagnosed a faulty valve - it wasn't going all the way to "W" so some hot water was going towards heating rather than hot water. So, in the end not something I could cure - always worth a try I guess!
Got a GasSafe engineer out yesterday who diagnosed a faulty valve - it wasn't going all the way to "W" so some hot water was going towards heating rather than hot water. So, in the end not something I could cure - always worth a try I guess!