That's my understanding Lebowski.
Fitton does not want to pay £xm off to the tax man think everything's rosie, only for the tax man to come back in a few months asking for more money.
He wants to know what is owed before taking official control of the club and like you say the HMRC cannot give these kind of guarantees.
And he wants to negotiate a payment profile that keeps the Revenue happy but enables him to pay over as long a period as possible.