Rather than use 10mm t & e into a 45amp mcb could I use two 6mm cables running into the one 45 amp mcb as I have a reel of 6mm cable spare?
No, use a 45 Amp Dual Appliance / Cooker Outlet Plate. If your using a 45A mcb, then ideally it should be 10mm cable. https://www.tlc-direct.co.uk/Produc...=adwords&kw=&gclid=CJ2quOXkxcUCFfMZtAodAQ4AeQ What you running?
If you could get away with a 32amp mcb 6mm t/e would be fine, subject to installation method of course.