Response to answer by SD. Thanks. That's helpful. Electrician tested heating coil by wiring it to mains supply and it is OK. Little blob of black plastic by cut out switch suggests it may have 'died'. If I wanted to find a 'retro fit thermal cut out', where would I look for one? I have no experience in this area but know there must be a solution.
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Try fixfault.com to fix convector heaters.