In reply to victorclimber:
To clarify, your solicitors will ask the seller's solicitors to get the relevant planning permission & building regulations information from the seller.
If the seller's sols can't provide the paperwork for the extension they should advise their client that they will have to pay out for an indemnity policy and that they as the seller should cover the cost of that policy. If they come back with anything else your solicitor should suggest that they do the above.
It's quite rare that the buyer pays for the cost of policies to cover the seller.