So that was done, and fortunately the tumour was removed with a clear margin and sent off to the lab who said "Oh, now we can see the whole thing, it's actually a Mast Cell Tumour, pretty benign actually." This was a relief.
Three weeks later, I spotted another lump - this time on Eth's side. It was big and black and the size of a pea, just under the skin:
The vets were worried - the pigmentation made them think "melanoma", but the size of it and the rapidity of the growth made them not want to have to wait another ten days for biopsy results. So back to the surgery with Eth, more general anaesthetic and shaving later, and the lump was removed. This one, according to the Lab Results, was an Apocrine Tumour - again, relatively benign, but good to remove. Since there are remarkably few pictures of Apocrine Tumours in ferrets on the internet, I feel I have contributed.