You could try getting a 45amp junction box, that would be the best way, having said that, I have only seen upto 30amp junction boxes but I suppose they exist
There are other ways, but they don't look very neat in a domestic situation. You could put a link box on the wall and join the cables in there. Link boxes are available for very large cables as used for industrial loads, so 10 mm sq would not be a problem. A cooker switch as recommended by others would look better, but could be switched off inadvertantly by somebody who didn't realise what it was for.
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