My son and I had a good look through the flock today and in total there are four birds with bumblefoot!!! Now I've looked this closely they certainly must have started this condition well before I went away. I feel quite ashamed that I didn't notice the swellings as I often sit and watch the birds while they go about their business.
The perches are removed every four weeks or so when I clean them out, washed well and DE slurry applied. The perches are 3x3 rounded edges, nice and steady in their fixings.
Orchardlady