I once bought a Microwave from Comet. What a disaster. It went rusty inside and they insisted on a repair rather than a replacement - 3 TIMES!! Eventually they said to call out the repair man rather than take it into the shop, who said to keep calling him out at £49 per call out, which I did till the guarantee ran out. It must have cost them several hundred pounds in all rather than £100 for a new replacement.
Bought my next one in Argos,a DeLongi, and its now 6 years old and I've never had a problem with it, and it was cheaper.
I've also bought many large electrical items from Amazon, with no problems.