netaheat 50 does not switch off?

I have a netaheat 50 which once switched on keeps running.
If I switch the power spur off and back on it stops but once the timer kicks on it keeps running when the control is set to off. It makes the radiators and water very hot. The only control that works is the circular control on the front of the boiler if it is turned down. Once the boiler cools a bit it fires up. Any thoughts? Thanks.