Today was the day to sort it out.
The problem turned out to be a seized bearing on the inner side of the hub.
I had to get a grinder and very carefully grind a flat on the inner race all the way through to the stub axle in order to remove the race as it had spun on the axle and welded itself on. The damage to the stub axle was only minor so after a bit of fettling the new race was fitted and all was ok.
While I was doing this I thought I may as well do the lot. The brake shoes were well past their sell by date, so much for the trailer being serviced just before I bought it!!
A good clean of all parts,
And the back plate
Then all was reassembled, adjusters and all nuts greased with copper slip, new bearings packed with fresh grease.
Brakes came with new shoes and all new springs so now adjusted and now all running free.