Third party core module repair?

Hello all!

I bought a FP4 second-hand, and all was well for a couple of months. The phone then completely conked out while I was abroad, so I took it to a little electronics shop and they said they think it’s the logic board. No issues with battery.

Unfortunately as I can’t get an original sale invoice Fairphone won’t touch it. Has anyone had any luck getting a third party inspection/repair of the logic board/core module?

In case it matters, I and the phone are based in the UK.

Thanks in advance!