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Thank you all for the advice. Unfortunately I do not own a microwave oven.
The potatoes I use are Tesco's baking potatoes - do not know their proper name, e.g. Maris Piper, Jersey, New, etc. We use a gas oven. The problem we have is that the centre of the potatoes are not always soft.
I cook the potatoes for 1-1/2 hours on a gas mark7 for an hour, then no. 5 for a half an hour. My other half, however, insists on cooking on a gas mark 5 temperature for only an hour.
Whoever cooks our spuds, itt is pot luck if they come out properly cooked!