Thanks for your comments and advice, apologies for taking a while to reply, I've been travelling to Montreal!
I have e-mailed Vaughns about the possibility of getting them to heat treat the anvil, but no response yet. I guess I'll try to find a heat treatment shop here in the UK who are willing to have a go? If I draw a blank, then I might try the shotpeening as a low risk treatment that might have some benefit?
I think that this anvil must have come out of Brooks by the 'back door' before it was finished? Maybe they had stock of castings when they went out of business? It's not cast iron and the casting of the makers name is very crisp, so I don't think it's a copy?
Thanks again.