2 people have cocked up here, the buyer and the solicitor.
The solicitor does not even get involved until the sale price has been agreed, at which point the tax due has also been agreed.
Personally I would never spend £250k + without knowing how much exactly it would cost me, and for all those above saying it is the solicitors fault , then sorry you are bloody mad if you will spend that kind of money without knowing the costs involved.
ok the solicitor made a big mistake when handing the tax over to HMRC, but they have apologised and compensated. The OP’s Mother is now better off that she would have been before the solicitor got involved, because when she put her bid in she would have worked out how much tax she had to pay.