Thank you for your replies. Turns out that there wasn't enough water in the system as the header tank had run dry because the valve had seized up and the tank was emptied last time we had plumbing problem and the plumber had failed to check that it re-filled and the tiny bit that was remaining...
Hi there - can anyone help? (Sorry if this is posted in wrong place). We normally run our hot water on constant but I turned the CH and HW off whilst we were on holiday last week. Came back and turned HW to constant again. Also turned the boiler down from 4 to 2.5 (top number is 6) as I wanted...