Suddenly this morning, I found the hot water flow through all mixer taps in the flat had stopped. The cold flow is OK in the bathroom sink/bath but the shower and the mixer tap in the kitchen (where the boiler is) are both not flowing (either hot or cold). The heating appears to fire up, warm the radiators then not fire up again after say 10 minutes. Is this possibly motorised valve failure? The boiler is a Vaillant Turbomax vuw 242