What a shock, Nic! When I took Brammie to the vet with the lump on her leg, the vet was pretty sure it was a tumour too, due to the shape of it. He was pretty worried because the lump was right over the joint in her leg and he wasn't optimistic about surgery.
Bramble's lump turned out to be a grass seed that had been there for 3 - 4 months and migrated up her leg.
Any chance of posting a picture of Alfiie's leg? It would be interesting to compare it to what Brammie's looked like.
Sending love and positive thought to Alfie and you. xx