The difficulty is that if the neighbour died intestate and there is no family, the estate may pass on bona vacantia (ie to the Crown). Even if there is an administrator (either the Treasury Solicitor or a relative) there is nothing you can do. You have no cause of action, other than to report any hostility to the police - even then they cannot evict the person who upsets you.
It could take AGES to sort out. I dealt with a case last week where the deceased died in 1994 and it was only sorted last week because of the problems tracing the beneficiaries (although that was an extreme case).