Get legal advice is the correct answer.(posted 8831 days ago)The reason they want you to send a letter of acceptance is because - theoretically - they have to inform you within 21 days of the sale and the terms of the sale. If you have kindly written to them confirming that they have informed you then you will have removed one possible argument against them in any subsequent shortfall chase.
In my opinion, you should not feel obliged to send letters at your own cost simply because you have been asked to do so. So I would be inclined not to do so.
Lee