Mike Lindsay wrote:
The overrun brake on our Comet trailer is jammed in the "brakes-
on" position, and has defied all attempts to free it.

I understand there are nylon components inside the trailer
tongue, so we don't want to try the time honoured option of heating it
with a blow lamp.

Does anyone have any suggestions?

I guess you've already tried hitting the wheel nuts with a lump hammer,
with the wheel jacked up so it can rotate if it frees. Works on my
trailer, but not if I hit other parts of the hub - no idea why.