I would say that teachers can be of 'variable quality', blackrock, whether they have a formal teaching qualification or not!! Many of the best, most passionate, intelligent and talented teachers I have worked with had no PGCE or QTS. Likewise, many passionate, intelligent and talented teachers I worked with did have a PGCE or QTS. Swings and roundabouts.
A PGCE is not 'necessary' for teaching within the independent sector (sorry if I'm repeating other people). I have just been browsing jobs on the TES website, and not one in my subject asks for a PGCE or QTS. They all, naturally, want a good degree in the approriate subject (for senior school teaching).
Thanks for the heads up about gaining QTS via the OU, Martianbishop - I'll definitely have a look into it! The reason I would like to have QTS is, I suppose, because it would consolidate my teaching experience into a recognised 'qualification'. I would hope to be successful having years of good GCSE and A-level results behind me, plus being in charge of GCSE coursework etc. It would also give me a bit of a confidence boost as once I want to return to teaching, I'll have been out of practice for a number of years (apart from exam marking).
IMHO, I would not be concerned with formal teaching qualifications, and I won't be when I look for schools for my DC (if we go down the independent route). I only have my seven years' experience to go on, but in that time I came across many, many teachers - enough, I think, to form my opinion!