The first bit of this post will seem to be completely unrelated to your question, but please stick with it. (It will, hopefully, make sense eventually!).
Back in the early 1990s I worked, on a 'commission only' basis, for a firm that claimed that they could get reductions in people's business rates because "we've identified probable errors in the way that the rateable value has been assigned to your property". The telephone canvassers (who were also paid only on commission) targeted businesses at random (despite the claim that they were aware of a particular problem in the area where the business was located). Potential customers were asked to pay a fee (starting at £125 for a small shop, for example) which they were guaranteed that they'd get back, in full, if the company was unable to secure a refund.
The appeals process was genuinely slow, so when people complained (many months after parting with their money) that they'd heard nothing about their appeal, the company would just blame the Valuation Office (even though many appeals wouldn't even have been started by that time).
Eventually, surprise, surprise (a few months after I stopped working for them), the company went into liquidation, leaving thousands of customers out of pocket (but also leaving the directors extremely well off).
I later found out;
(a) that the directors had operated several similar businesses (where they claimed to be able to get people refunds from councils or from Government bodies), with exactly the same outcome ; and that
(b) that company was only one of dozens that did exactly the same thing.
Now, I wonder if you can see why alarm bells started ringing in my head when I read your post?
My advice is that you should definitely not touch the company which approached you, even with the proverbial bargepole. Then you should try to find out exactly what Stamp Duty Land Tax you actua