This is their website i think -
http://www.impact-london.co.uk/index.phpThey approached a load of people (including me) in my year at Drama school after our showcase and invited us all to audition for them. I think only a few of them actually went but no-one ended up signing with them so can't really tell you what they're like as an Agent I'm afraid - their website doesn't list their clients or CVs - just 'Major Musicals' their clients have been in (but they don't say which productions - West End, Touring or Abroad?) so if you go to meet them I would be sure to ask exacly what sort of work they have managed to secure in the past year or so; that's usually a good way to gauge how hard they will work for you.
Also, if you end up signing with them, make sure you're really clear about the sort of roles you want to be put up for and what your strengths are. I have several friends who didn't make that clear when they signed with their agents and it led to a lot of grief further down the line. Of course, they might disagree with how you think you should be cast, but make sure you tell them what you're aiming for and you can go from there.
Hope this helps!