Item 261751134767. It went for £1.75, my snipe was at £5.10 but it never bid. Can you tell why?

I checked the logs and it appears eBay was slower than usual to respond, so the sniper did not have enough time to resubmit your bid.
This is rare, but happens sometimes. I'm sorry it affected your snipe.

Thanks for checking this. Would changing the "My Time" to more than the 5 sec default reduce the chance of this happening at the cost of increasing the chance of someone else sneaking another bid in?

Yes, it will. That's why you have the time settings so you can adjust accordingly to your acceptable risks.