Thanks for your reply and apologies for the delay.
Well, the good news is that even if the Freeholder does sell, the new Freeholder is still liable for any claim you have made against the current Freeholder. As you have evidence that you have written to the current Freeholder, there will be no dispute over your claim.
Furthermore, if the Freehold is being sold, it would be usual for the Buyers Solicitor to ask the current Freeholder's Solicitor if there are any outstanding claims or disputes with any of the Leaseholders. If the Solicitor discloses the claim you have made, it would also be usual for the Buyers Solicitor to ask that any claim be settled before advising his client to proceed with the Purchase.
As you have also contacted the Freeholder's Solciitor directly, they would be breaching their duty, if they did not disclose it to the Buyers Solicitor.
If your dispute is not disclosed, and the new Freeholder only gets to know of the claim from you, after he has purchased, he can sue the current Freeholder's next of kin/administrator for breach of Contract.
It may well be a blessing in a disguise that the Freehold is being sold!
I hope this helps and sets out the legal position.