Cramps/pain in feet and toes after turbo trainer rides and cyclocross races Recently I purchased a pair of mtb shoes one size larger than I needed (due to lack of availability and time), sizes I needed: uk 11 the size I purchased: uk 12.

After a turbo trainer rides or cyclocross race I have cramps/pain in feet and toes.

Should I replace the shoes or could it be something else?
It's almost certainly the shoes but exactly what is a really variable question. New shoe problems tend to fit into the following:

1, Does the shoe fit properly (length and width)? Seems the answer is no as you got a size too big which means your foot will move a bit. This can be mitigated with the correct insole and you can even get mouldable ones which you heat, put in your shoes then put your shoes on.
2, Is the insole right? Loads of shoes come with an "average" insole which doesn't really suit everyone. My road shoes came with 4 sets of inserts to tune the fit properly and it's noticeable when I go back to the cheaper ones. Consider transferring your old insoles if they fit or get some new ones
3, Is the cleat position right? Given the shoe size is wrong, this will be hard to get right just by eye. I tend to take a few hours riding with a pair of shoes to get it approximately right and then maybe another 20-30 hours to get it exactly right. After that I either use the system the shoes use to retain the cleat position when I put new ones on or the low-tech approach of a marker pen.

I'd do 3, then 2 then if that fails and your feet still hurt, get new shoes.