"Also if she can not find her tenancy agreement to prove she has an assured tenancy then obviously you have grounds to get rid of her."
This is wrong: Pre-1989 - regulated tenancy, 1989-1997 - assured tenancy, 1997-present - assured shorthold tenancy. At least by default.
This tenant moved in in the 70s! So there is no chance that this is not a regulated tenancy.
And there is no distinction between a verbal and written contract, so that's completely irrelevant.
Unless the landlord can prove otherwise, he will be bound by the law:
(a) NOT to evict the tenant except in very special circumstances
(b) to maintain the structure of the house.
The remedies for this are:
(a) approach the tenant and ask how much they want, in cash, to leave (will be tens of thousands at least)
(b) sell the house at reduced value given the sitting tenant
(c) give the house to the tenant.
(d) wait for the tenant to die, but in the mean time maintain the fabric and structure of the house by whatever means
Eviction is not an option here.
Obviously it's a shame that this inheritance isn't worth as much as a vacant house, but it's still going to be worth more than was paid for it in the 70s....