What exactly do they mean by access to your estate agent, it may only be to see that it is being sold and get a realistic timeframe for sale.
The sale will cost c 5% + of the proceeds and in some cases in the current market there the buyer expects the seller to pay his costs also which can be 10% +
Depending on the values involved they can register a charge against the property or/and can apply to the courts to have a lien put on the proceeds. The costs of all this will be for your account so if the apt is small beer then they may not bother
Have you engaged with them along the MARP process Mortgage Arrears Resolution Procedure this gives you some protection if you can reach agreement