It's a common issue on the Fiesta.
Water drops down onto the mechanism and that's why the boot won't lock and keep popping open.
Take it to a dealer and they will sort it for you.
Hi James, I had the same problem with my MK7 Fiesta. Its the boot lock switch. Water gets in through the rear wiper and drips onto the switch and wires. I paid about £60 when I took it to ford. Hope this helps Cheers