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All Shimano 8/9/10 speed will work on 8 speed systems.
You could get a 105 5700 RD for cheap on eBay.
Or this new Shimano Sora RD 3500 9 speed for £14.84 delivered:
http://www.wiggle.co.uk/shimano-sora-9-speed-rear-derailleur-1/8 speed parts are quite affordable so you could push the boat out and get a new cassette and chain while you are at it - they are about £20 for the pair on CRC/Wiggle and it saves you cleaning the current ones ;-)
Has anyone got an 8 speed Shimano rear mech (that will work with indexed shifters) they don't want?
Mine snapped in two after a mere two years and I need to get the bike mobile again.