I wouldn't spend money with a solicitor just yet.
I would complain to him in writing making sure you state all the facts ie when he stopped being your agent, when you first asked him to remove the details of your house and how many times subsequent.
Send this off in the post and in the meantime find out if he is a part of one of the Auctioneering bodies. If he is send off a letter of complaint to them with a copy of the letter you sent to the auctioneer.
If he still doesn't remove it after this I would send him a letter warning that you will be contacting your solicitor to deal with the matter.
I think if you do this you, you may be able to recover any costs from the small claims court(if you wish to), as you can prove that you gave him every opportunity to remove your house details from his window.