Rosieres cooker won't heat up in normal time...can you help?

My Rosieres cooker is 18 years old and I only have to fix it once, over 10 years ago. It's look almost brand new and I really don't wish to change it but in the last few days my right oven takes over an hour to get to 150C and all knobs get very hot....left fan oven is o.k. but it's door gets very hot too. Can you help me? I wonder if the temperature termostat is broken? I live in Beds/Bucks and Herts any repair services known?