Lawyers don't – as a rule – like to sue their clients for non-payment of bills.
So, go back and say you're not happy with it, and offer an amount you would be happy to pay. Then agree the amount you'll pay and you're done.
If it does become difficult, then you can go through the Law Society, but in the meantime remember that (1) you're trying to pay what you think is a fair price for the work done and (2) the lawyer is trying to make a living (though if looks like he's trying to make a killing, then get the boot in).
(I don't think I do a deal where the fees – even pre-agreed fixed fees – aren't queried at the end. The worst was a project I worked on where our original fee quote was £250k, but we billed around £10m. I believe there was a bit of a "discussion" at the end….)