My guess?
Very elderly gentleman's residence, died in squalor years ago, probate took years and the house was put up for sale as soon as the Tax Man came calling for his Inheritance Tax.
Jarndyce v. Jarndyce style proceedings have caused the house to be neglected, there are few ceilings left, the floors are death traps, the bathrooms are barely functioning, some relative or other has tried to claim the house as theirs, but they've been hamstrung by lack of cash preventing them from buying out the other claimants and the Listed status has prevented a lot of work.
One or more relatives, buoyed by Location, Location, Location/stupid London prices have overpriced the property, thinking it's an easy sell due to location and previously sold prices from around the area.
The estate agent thinks these people are complete morons and has probably tried to persuade them to reduce the asking price, but the dopey fools are clinging on to the hope of a life changing sum of money to recoup their losses on legal fees, taxes and retirement.
Just a guess.