Replace cooker mounting box

I'd like to do this work myself, but haven't done much elec work before. I realise I should probably get help - but would like to ask for your advice first.
The old cooker mounting socket is 35mm, set in breeze block wall partly, but protrudes as there used to be wooden splash back thingy (15mm)lining the walls instead of tiling - but now I've removed the wood and want to tile! Should I try and dig deeper into the breeze block, or disconnect wires and swap to a 25mm mounting box? How easy is this? Anything I need to be careful of? Or should I say, precautions I should take, apart for switching off mains!
Many thanks,
John

What is the cooker mounting box for?

Is it for the cooker control unit or for the cooker outlet plate?


cooker control unit:


cooker outlet plate:

SOrry, it's for a cooker control unit. Big red switch for cooker, plus additional 13amp socket for the toaster!!!!

You are probably better off sinking the existing box into the wall as the cables will be quite thick, and you may struggle getting the switch to screw back properly into a 25mm back box.

Thanks, that confirms my thoughts. You're right, the wires are very stiff, and I wondered whether it'd fit into a 25mm, plus I wasn't too keen on disconnecting them. I'll get the drill and chisel out instead!
Cheers,