First of all, I have to admit I'm not an authority on the older assured or protected tenancies as there are so few around these days but, based on my knowledge of the newer shorthold tenancies I dont think the landlord can simply let the property deteriorate around the tenant. With a more up to date tenancy you could get 3 quotes for the work and choose the best and get the work done yourself and deduct from the rent (might take a while I understand if its only a nominal rent) or ask the landlord to recompense you and eventually taking them to Court if they dont.
I have to stress this would be for an assured shorthold tenancy. I dont know if the agreement your mother has would change this. I dont often recommend Citizens Advice as they sometimes give very poor advice but it might be worth trying them.
�800 does sound a bit steep unless its a very big fence.