Glowworm Spacesaver KFB 60 overheating/cuts out?

If I have the hot water on the boiler for 20-30 mins the boiler is fine, I can also start the central heating cycle after if I wish to do so.

If the hot water is on for 40-60 mins my upstairs radiators become hot, even though central heating is not switched on. When the water cycle finishes it cuts out the boiler so can't start central heating cycle or switch boiler on until boiler cools down. Is this due to the system overheating??

If I have the heat on and have the water cycle on say for an hour during a 3 hr heating cycle I have no problems with the boiler switching off. It doesn't appear to overheat when the heating cycle is on first.

I would be grateful for any advice on the cause of this problem and possible solutions please. Many thanks in advance.