Thanks all, I'm reluctant to buy a new pair of shoes at this late stage as I have precious little time to bed them in and they may create more problems. However, I'm formulating a plan
Buy some Specialized blue inserts for my shoes, shaping them to fit the perimeter outline of my shoes.
Ensure I ride from the start with loose straps at the front.
When I stop at controls take shoes off (was going to do that anyway)
Take some yoga mat foam, scissors and tape with me so I can fashion further inserts if needed.
The final one and more radical is to perforate my shoes to increase airflow in an attempt to reduce heat induced swelling.