There are sometimes clauses in property sales, especially if it is leasehold not freehold property.
however I cant see how they could force this upon an owner unless one of the following applies.
If the owner is renting the property out there can be action against him (the landlord) for not repairing electrical faults in a timely manor.
if it is a property in a block, such as a block of flats, where a fire or other electrical fault could make others escape routes or property at risk.
If it was purchased on one of the schemes where property is sold off cheep with a requirement to bring it up to a certain standard within a certain time frame.
I would advise to keep all communication in writing, either by post or email. (if an email is not replied to, follow up by printing it and sending in post)
it is easy to get passed from pillar to post with large organisations, especially councils and other government departments, if you are trying to deal with it over the phone.
also, i would not be spending money on solicitors at this point, hold your ground.
make it clear that you are not preventing them access to one of there properties, they are asking for permission to enter your own property and permission will not be granted at this time or in the near future.
if they still want to pursue it, they will send you a summons to court.
This document will have to outline the case they have against you and what law they are using to force entry.
this will give you the info required to have it thrown out of court (assuming there are no caveats in the house sale documents)
p.s. the courts are unlikely to force entry to a property containing a vulnerable person for non essential work when it is against government guidance for them to be interacting with people. even if they had a strong case.