What the mortgage company is asking for is standard practice - i.e. independent valuation using one of the bank's nominated estate agents, filling in of SFS form, either face to face meeting or telephone interview with mortgage advisor etc. These are simply steps that the bank must take. I did it myself when selling our home on behalf of the bank. They may seem daunting, but they are all manageable. An independent valuation costs approx. 150 Euros.
The 'Agreement to Sell Form', will also likely include a section where you will have to suggest how you intend to repay the shortfall (difference between selling price and what you owe). It's important to not 'over promise' in this section.
It would be impossible for the bank to force you into selling the family home as your ex has equity in the house. Besides, it would cost the bank more than the approx. 4000 shortfall to bring you to court in the first place.