A quick update - almost two months later, it's fixed. Customer service were slow to respond, it took an age (more than a month) for the parts to reach me, and then I waited 10 days trying to find a soldering iron. In the end I swapped out both fans without one. It was pretty easy, only dodgy bit was burning the wires together with a lighter. Appears to have worked in any case, and I can start cracking on with my fairly sizeable backlog. I think next time it goes wrong it might be time to replace completely...