Dimplex Bathroom off peak heater?

Storage heater in Bathroom XL56 is not working, every time it should come on it triggers the main switch on the main fuse box. It was installed by Scottish Gas 17/9/08 however we did complain long after that date that the heater was not working right. Our customer reference Number 65226. Can you advise me is it still under guarantee, if not what is your call out charge ?
We look forward to hearing your views
Anne
I tried phoning 08456005111 but could never get through.