
Sounds like a harmless cyst to me, but we do get a bit paranoid with our flatcoats as soon as lumps appear. I suppose you could ask the vet to do a needle biopsy and send it off .
The Flatcoated Retriever Society have a scheme for assessing tumours free of charge to members, but they do need to be excised completely in this case. I can give you the contact number in Cambridge vet school if you need it.
Have found ours come up in odd lumps occasionally when they're due in season - sort of hormonal, I suppose.
Jo and the Casblaidd Flatcoats