Hey MarkT.
This sounds pretty convincing to me. I will try to get hold of some shielded twisted-pair cable and add the 100pF capacitor.
Do you think my idea to reduce the resistor value in order to make the problem more reproducible is a good idea? Basically I will swap the 1k for 10k, check that the cap and cable fixes the issue still then switch back to 1k.
Now I just need to get hold of ~2m of this cable here in the UK! I'll get back to you guys once I manage to do so and install it.
Thanks again!