To clarify, the vet had been on holiday, and was very apologetic for not replying earlier.
This is an old ex-battery hen, no longer laying, and has presented like this for about a year now.
She's a real character, as has always had an awkward gate (long before the condition was noticeable), and as awful as the condition looks she still has a good appetite, and appears happy in herself, etc.
I could go for a 2nd opinion, but consultations are expensive, and it's a well established countryside vet who I'd hope would be experienced enough recognise a tumor/cyst?
The vet did say that there was a probability the tumor would grow further and the skin would eventually spilt....and that would be the time to consider having her put down (assuming she remains happy in the mean time).
I assume she is just being realistic in terms of balancing treatment implications and cost, with the age of the bird, etc.