Worth finding out if you have any motorised valves and where they are. They can get unreliable with age but changing the motor or head is easy if you are comfortable with electrics.
Thanks, I'll take a closer look once ms_com gets up. Hard to diagnose faults with something you don't fully understand while a toddler screams at you for doing other basic things wrong.
Just read a forum post somewhere else that talked about exactly that. Although since I turned both HW and CH on this morning, rads are heating up.