Would the estate agent and vendor be aware of that restriction in the listing I wonder? Perhaps someone should inform them before it ends up in the skip!
Bargain indeed, though for a reason. As a listed building but with major structural problems it is only being sold to cash buyers with the means to effect the repairs necessary, and I should imagine that the cost of the building itself would be dwarfed by the cost of restoring it to its former glory. The present owner (himself a noted architect who worked on the conversion of part of Somerset House in London to become the Courtauld Institute in London) sold off much of the surrounding land a few years ago to build housing in an attempt to raise more money to save the manor house, but it sounds as though far more is still needed. And in selling off the surrounding land for a housing development, it has inevitably reduced its value further. Just being a listed building would put a lot of people off buying it (including those who might object to having a pipe organ in the attic!) due to the additional costs of having to do any changes sympathetically.
Still, for the same money you could have a bedsit in central London and I know which I would rather have if I had the means to buy it!