I've heard some horror stories surrounding these rates assistance companies. Avoid IMO.
I successfully challenged our rateable value myself and it was really easy to do with the councils assistance, bit of a faff but worth the effort.
1) I compared my rateable value to similar forcourts/sites in our area online via the councils website/planning portal. Found out we were paying more per sq metre/foot than others for comparable surfacing (gravel, tarmac, hardstanding etc attracted differing rateable value for some reason).
2) Downloaded that info, drew a kids sketch birds eye view of our site with a pen with annotations and sizes, got my old rates bills and chucked it all in a scanned document and sent to the council simple covering letter explaining the issue.
3) Matey from the council came down and couldn't have been more helpful. Given the reputation councils have I was surprised and it seemed like they really just wanted to correct their mistake.
Remember to work with them not against them and have all your documentation physically to hand and in a PDF or something you can email.