At the very least, you should make a complaint (in writing) to the firm in question and the Law Society.
I'm not sure if the solicitor's actions have any impact on your liability to Revenue-you still owe the stamp duty that would have been payable at the time regardless, but you really shouldn't be liable for any interest or penalties that arise as a result of the solicitor's incompetence.
I'm not sure if it is possible to take a case to the Small Claims Court if you suffer financially as a result of this error, and no compensation is forthcoming from the firm in question.
I'm sure that one of the solicitors who contribute will be able to give you a clearer picture of what are your rights/obligations in this case.
Post crossed with ClubMan's-and we appear to be saying pretty much the same thing.