It's certainly a ridiculous situation Tony, made worse by their digging their heels in over insisting on carrying out an inspection at the premises, which as we've said in previous discussions is not mandatory as you are within your rights to refuse them entry to your property.
I would say this is a fairly standard 'scare' communication to panic you into doing something you've already said is unnecessary many times over, to avoid them taking you to court - in other words it's to save them the trouble and expense of doing so and having to prove that you're evading payment. The thing is that if you absolutely ignore this latest missive they just might be able to turn around and use that against you as proof that you're being deliberately uncooperative and evasive and so court action is now their only recourse to their proof that you need a TV licence.
If I still worked at a county court and this was a debt company solicitor's panic letter I'd suggest you respond to them online as it says on the form to confirm your stance and telling them to do their worst if they still insist in not believing you're rightly exempt from needing a TV licence on the simple grounds that you do not own a TV and have, as is your right, merely denied them access to confirm this on an eyeball basis. By doing that you take away their last card by facing them down and showing the district judge taking the case that you have gone to extraordinary lengths to 'evade' payment by simply telling them the truth all along - that you don't need to have a TV licence at all. Any person who was truly dodging the licence fee wouldn't have resisted this long and would either have caved in and bought the damn licence or let them in after chucking the TV out pro temps if they were hardened dodgers
Just because you happen to live in flats where other people have TVs doesn't mean you have to let them poke around your home on their say so because they think
you're telling porkies...