I wonder if anyone can advise me? I have the panasonic gf3 with a 14mm lens. Is there a macro conversion lens I can buy (apart from the new panasonic one) - haven't had one before and so wanted something not too expensive that I can play with. Also, what do people think about extension tubes? Any advice would be great - thanks
As attractive as a "macro converter", or even extension tubes, might be from a cost perspective, your 14mm lens is not a very good foundation for macro photography. It is a moderately wide-angle lens (equivalent to about 28mm on a full-frame 35mm body), so if you want any real amount of subject magnification, you'll need to be working in very close—your lens will be as little as 15-16mm (about 5/8") away from what you're shooting when you're in very tight. That doesn't leave a lot of room for light to get at your subject at all.
There are a couple of options for macro lenses, including the Panasonic/Leica 45mm lens you've mentioned (at around $1000) and an Olympus 45mm 1:2 macro (at around $400). They're great if you want auto-everything, except for that unfortunate money thing.
If you are willing to sacrifice automation, you may find that using a lens mount adapter plus extension tubes and grabbing a 50mm lens (of just about any make you can find an adapter for) might just be a better solution. Since the μ4/3 sensor is fairly small, you probably won't need to go fully life-size on the sensor very often, so the 50mm focal length (which would really be too short for 35mm or APS-C format macrophotography) will still give you plenty of working romm at relatively large magnifications. 50mm lenses in the f/1.8-2 range are plentiful and cheap out there, especially for older manual-focus 35mm cameras, so finding one used at a very reasonable price shouldn't be any problem at all.
It means more work for you as a photographer (manual focus and exposure), but the results are likely to be significantly better than trying to do macro work with a wide-angle lens.