How can I fix my Sabre HE Boiler?

Hello

I have a Sabre HE combi boiler, about 2 years old. We rarely use the heating (probably last put on in February) and just use it for hot water.

Today we noticed the radiators were hot. The one next to the boiler is absolutely scorching. The switch is on Hot Water only.

If we turn the switch to off, they dont cool down.

If we unplug the boiler and wait for them to cool down, when we switch it back on in the Off Position they do stay cold. But if we select Hot Water or HotWater/Heating the radiators come on permanently and no hot water.

Any suggestions?

I am a technophobe so any suggestions will need to be in incredibly Plain English!

Thanks