Ailz and I didn't like the sound of this at all. We did some research. The outfit who made the call are called Dalton and Dalton. They have a website. If they manage to get you a rebate on your Council Tax they claim 25% of it.
Thing is, anyone can ask for a review of their Council Tax banding. All you have to do is ring up the Council's Valuation Office and the Valuation Office does the rest. Dalton and Dalton may be operating within the Law but what they're doing, when you boil it down, is charging 25% of what could turn out to be a fairly large sum for making a single phone call.