I can heartily recommend this guy, based in West Norwood. He's come out twice, to fix different part of my cooker, recently.
http://www.cookersolutions.com
He doesn't mark up on the price for components. And, if he can't fix the problem because it's not economic to repair your oven, he only charges a small call out rate for diagnosing the problem - which slightly sugars the bitter pill of knackered kitchenware!