Hi Steven ..
Apols for delay in responding .. only just seen this ..
I think the bird(s) have some kind of infection (possibly Mites) .. there have been a number of reports about similar bald patches on Robins & Finches (particularly Green Finches) lately ..
If they visit your Bird table or Feeding station, I'd suggest regular cleaning with a mild disinfectant solution & always wash your own hands afterwards ..
Cheers for now ..
Kev ..