Revision courses by independent companies - comments?

[east-of-the-Rye]

I (and my son) are thinking of putting him onto a couple of 3 day Easter revision courses in Maths & Chemistry to help him get through his A-levels. Companies out there include Excel in Key subjects (which we are looking at in particular) but I'm also aware that Justin Craig and others run course. Does anyone have any knowledge about any of these companies and how helpful they are?

thanks.