I have now tried crispy coated garlic mushrooms and battered onions rings from the frozen food section in Sainsburys. Both come with instructions for a 'normal' oven with a proviso for fan assisted ovens that I translate as 'Your on your own with these'. Both suggest a time with a pre heated oven temperature of 220C.
So far my best results have been obtained at 180C on the low rack in a baking tray in my H oven. The timing seems to be the critical part of the cooking. The difference between a cuttable coating and needing a sharp knife is where the art of cooking is needed. The time required for best results is significantly less than given in the instructions on the packaging e.g. 18 minutes becomes somewhere between 11 and 14 minutes. Turning over the items half way through does seems to be a good idea.
Does any one else have frozen food experience?