I've been swithering about embroidery frames. Closetmonster knows. Anyway, I came to a decision and found a cheap online shop which supplied the kind I want to try first.
http://www.siestaframes.com/acatalog/siesta_bar_frames.html You buy pairs of 'bars' in whatever size you want, and then fit them together to make the square/rectangle you need for your work. They come in a huge range of sizes.
I got five pairs of bars for £6.85, plus £1 for P&P. (Their DMC embroidery threads are 49p a skein BTW.)
http://www.needlecraftexpress.co.uk/ That's cheaper than some of the individual frames I've been looking at - let's hope they're worth the money. I should have looked for reviews but I was overcome by impulse.