OK so we tried a PNG sequence. The problem was even more apparent. We tried no MP and the issue was the same.
So we took off effects one by one and we solved the issue.
Still doesn’t explain why it works on Mac but not PC. To be honest, it’s been a nightmare. The PC does weird things that the Mac just doesn’t do – I now get the saying “Macs just work”!
The latest error is:

We needed another machine and just ordered a 12 core Mac Pro. It’ll be interesting to see the difference!
Thank you for your help btw – always appreciated.
Olly Lawer