why do people spend so much time saying ' foo I dont know, but Mrs mopp darn the road may'
https://www.bloomberg.com/news/articles/2022-09-23/uk-stamp-duty-cut-when-will-it-start-how-much-is-the-reduction?leadSource=uverify%20wall
answer to day
Your are a third party and have not mentioned the two important dates - EXCHANGE and completion.
Singing the contracts does NOT mean they have been exchanged.
The important date was completion - the rule "pay after x days" - the clock started on completion.
The tax was collected by the buyer's solicitor and I am sure he would not collect a tax which was not due.