The cable sizes look to be 6mm and 2.5mm.
The 6mm will typically come from the cooker switch at bench level to the outlet (the part you are trying to change)
You need to work out where the 2.5mm is going as it's not a usual part of the cooker circuit.
It may be the case that it's not needed and all you need to actually connect is the 6mm (which should fit the socket outlet you have).
Interesting. If I don't need it, can I just tape it up and ignore it? Or should I remove it? There is a plug socket to the left of the 'cooker cavity' which I suppose it could belong to and hopefully should be easy to figure out (I'll just unscrew the switch, and tug the wires and see if it moves I suppose). I would then just replace that socket with a blanking plate.
I'm going to buy a different brand of switch which looks a bit more accessible and try that and if that doesn't work then I'll try the above. Again, thanks for your help! Once I get this all sorted, I'll paypal you and Adam a beer!