I'm sure there is Council Tax fraud and a bit of robust checking up is perfectly in order. But if the firm involved is "reputable", they will have no problem providing the recipients of their threatening letters with an address. We have to assume - in the absence of an address - that something dodgy is going on.
Then there is the tone of the letter. Gone is that quaint old British idea of public servants at your service. It's send this back to our undercover base in Twickenham or you incur an automatic fee of £70. As I say, an old lady frightener, that one.
Then there is the question of can they be trusted? At the bottom of Page 2 they say they will share your information (that you have provided in order to avoid handing over £70) with everyone, which is another way of saying "dont trust us" in so many words, is it not?
Then there is the sick comedy side of it. Tax exiles engaged in tax collection.