I can only echo the warnings above about companies going under just after you've paid them a handsome sum! It happened to me a while ago (nothing aviation related) and I learned the hard way that unsecured creditors are at the bottom of the list!
However, saying that I have been going through my PPL by putting down sums of £1k at a time as it allows for a certain discount of the price of lessons. Before I did this I checked the company accounts were in order just to give me piece of mind!
Try
www.companieshouse.com - you can find general info about the company and can order certain documents. I would recommend getting the latest full accounts and returns filed for Cabair before putting any money down. Cabair have a number of companies within their group, check them all if you can (it will cost £4 per company but nothing compared to what you could lose) - they are probably all guaranteeing the other. Just remember though, that this method is not fail safe - companies can go under in a very short space of time, but at least if the accounts look fine, it is a lot less likely!
Good luck!