This should have been picked up by the solicitors that acted on the purchase and the solicitor would normally have sent you a plan showing the extent of the land you are buying and asking you to confirm that what’s on the plan and what’s on the ground are the same.
The relevant period for adverse possession is 10 years, not 20.
I don’t know whether there is an allegation that the solicitor is in any way at fault here but of the solicitor is refusing to deal, you might want to make a formal complaint in writing to the firm’s complaints Partner.
If the Seller agrees that this piece of land should have been in your title, then it’s relatively easy to resolve although it’s not the solicitor’s fault, they may want paying for it which is why they are being uncooperative.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have