Hi Spartan1,
I can understand why you might be annoyed, but consider this:
1. When you say you want to 'go after' a service provider for shoddy service, you are in reality talking about suing for compensation.
2. In this case the service provider (your solicitor) has admitted the error and has undertaken to compensate you, by paying for your losses.
3. The stamp duty liability is not a 'loss'. It was your liability. It is annoying not to have known about it beforehand, but that annoyance does not actually constitutute a financial loss. The only conceivable way you could argue that it is a loss is if you were to say 'well, had we known there would be stamp duty, we would not have gone ahead with the transaction'. I am sure you can see that such an argument would lack all credibility. If you had not gone ahead with the transaction, you would be far worse off today.
Your solicitor was negligent; he admitted it and undertook to make it right. Mistakes happen; it is very annoying, and you are right to be annoyed. But I think your solicitor has acted very properly. Provided this was a genuine once-off error and not an indication of systemic problems, if you stick with him, I think you will probably get very diligent service from the solicitor in the future, (certainly any time I have a screw up on a file, i make double double sure not to have another one for the same client) so don't make any hasty decisions.