Firstly I'm sorry for your loss.
The death of a close relative can be a trying time, especially a parent.
Has your mother also passed away or else is otherwise not in a position to clarify this matter?
Regardless, while your father may have been mistaken you have to take on board his comments about his solicitor.
Given that the solicitor does not have the deeds which your father understood he had, he may not be the best person to seek advice from.
If he has lost the deeds or failed to register them if he was involved in the transaction and if he was required to do so, then he could be liable in any action you might take to seek redress.
In what sense therefore did your Dad by the lands?
There must have been some sort of contract and exchange of monies.
You could start by following the money, but the records may be old or lost.
You could approach the vendor, but he may have no obligation to help you.
I'd start the ball rolling by writing to your father's solicitor, setting out what you know and get a reply.
If you get teh same denial I'd then be tempted to retain your own solicitor to correspond with your father's solicitor.
There are too many tales of solicitors playing fast and loose with the trust of clients these days to simply allow his denial stand.
At the very least, assuming he was your father's solicitor at the time the transaction took place, he should show why he wasn't involved in it.
HTH
ONQ.