Hi chaps can anyone tell me what diameter cable i would require to run from the battery at the rear
of the car to the starter.

Cable run would be 5m from Odyssey battery, to standard 2.0 Zetec

tia Paul

That's a fairly long run! At a rough calculated guess, I'd say 50mm should suffice. You could go to 60mm, but the lugs on the end don't come in that size - only 50 or 70. If you buy decent cable it should be about £7-8 + VAT per metre, and be relatively flexible. But as a precaution, I'd also get some split conduit for extra protection.

5 meters is a very long run, even my mondeo estate would only be 3.5 meters at the most

for that length, you should run an explicit 'earth'/negative cable too
if I were buying cable for that I would consider
https://www.amazon.co.uk/1200amp-leads-booster-cables-long/dp/B009SDD9LM
at about £23

5m was being quite generous ,im sure i can smooth the path out as well.