Thanks for your reply and apologies for the delay.
From a legal point of view, you did of course contract to sell the property to your Buyers and as the Contract was fulfilled (ie they paid you the agreed amount), you are bound to transfer the title to them. Accordingly, they could sue you/take action if you did not sign the necessary paperwork.
From a practical point of view, their Solicitor has obviously messed up and they will want ot resolve this matter as soon a spossible, especially if the Buyers are now selling. Until you sign the Transfer, their Sale will not be able to proceed, and they won't want ot issue proceedings against you due to the cost and delay.
In this regard, you hold all the cards.
In these circumstances, it would be usual and reasonable for you to sign and return the Transfer, without receiving any compensation (on the basis that you are legally bound to do this, in accordance with the terms of the Contract), but provided the Buyers pay your legal fees involved. I am surprised that they have not already offered this to you.
You could try and hold out for some "compensation" but legally you are not entitled to any. However, I am sure the Buyers would be more than willing to pay your legal fees, just so the matter is put to bed.
I hope this assists and sets out the legal position.