JHM,
There is nothing wrong with the instruction you'll get from Cabair, nor anything wrong with learning to fly at Elstree. The quality of the instruction, their ground school materials and their facilities, in my experience, is pretty high.
You should know that it certainly isn't the cheapest option though. The planes they use tend to be more modern than most places, plus they charge extra for everything. For example, they charge you landing fees at Elstree when in reality landing at Elstree is free for planes based there. If pushed on it they'll tell you that it's not a landing fee but their handling fee, which frankly I feel is a bit of a scam. Being a larger school means they've plenty of planes in case one is out of service on any given day, ditto lots of instructors. They are unlikely to disappear overnight which might be a concern with a small school. They don't have much of a club atmosphere, so if that is important to you then you should look elsewhere.
If you want to take a free trip from Elstree send me a PM.
Keeping a PPL current - extremely variable, depending on what you want to fly and how much. Basic hiring is going to cost you about 100+VAT an hour. If you only do it once a month you'll find you have to keep getting checked out (i.e. 'mini flight test') by an instructor, so you probably want to aim for every three weeks or so. So, approximately £2,000 for the flying, say £200 for landing fees, £100ish for revalidating your type rating (200 every 2 years), £100 for charts and flight guides and stuff: about £2,500 a year. If you've a few thousand pounds to put down you could join a group and pay perhaps £75 per hour and £50 per month fixed fee. Or fly a microlight or PFA type and pay much less. There will be people who say that flying only 20 hours a year is barely enough to keep you current, but it's down to the individual, both in terms of their own abilities and desires, and what they feel comfortable with.