Just a quick update to everyone here as a thanks to those who replied.
Yesterday, between the rain, I found a nearby scrap yard that was breaking a 400h and went there. I tried a relay, and it wasn't the relay.
So when I took off the panel to try a new motor instead, it turns out that a cable (I think the one that goes to the rain sensor) had broken off and got completely tangled in the mechanism.
As soon as I untangled the wires the motor started working properly again.
I'm not sure what damage being jammed all of that time did so I replaced it anyway (£30).
Now I need to look at how to fix the rain sensor. I sense a new post coming up...