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Thanks Buenchico!
After reading the free software reviews I checked the comments and that led me to D3DGear, which looks OK and doesn't cost much.
Still surprised by file sizes even though it's much better than Fraps. One quote is 264Mb for 5 minutes (which in Fraps gives you a matter of seconds). It may be a case of "That's what you can expect" but on YouTube I've watched HD gameplay walkthroughs of 10 hours. How the blimey does anyone upload something that long?