Good morning, my name is James.
If your not getting an error shown on the microwave, but it turns on and the timer counts down etc, this would suggest a fault with the magnatron.
Please give me a few minutes to look up the cost of a replacement part.
Sorry for the delay in getting back to you, it appears the unit and parts are discontinued, I have tried all the usual spares companies such as eSpares, PartMaster and Russell Hobbs direct.
The magnatron is generally the most expensive part of the microwave, so if you were able to find a engineer to come out and fit a second hand one your probably looking at over £150 for the second hand part and labour.
Due to the difficulty sourcing the part, this is looking at being uneconomical to repair.
However I have managed to find the same model on ebay going for under £50 here, which may be of interest to you.