For £40 your best bet is a second-hand camera shop; I would not buy a second-hand one off eBay as it is not guaranteed to work. However, for just under £70 you could get a Metz mecablitz 36 AF-4 brand new. The only problem with this flash is although it extends, it does not swivel. Thus there are limitations when shooting in portrait/vertical mode. Depends on where you live, but this is a good link to shops that sell second-hand equipment in the UK:
http://www.ukcamera.com/usedcams/index.htm
Other brands that make flashes for the Canon system include Nissin and Sunpak, but both at about double what you have to spend. You could also pick up a Yongnuo Speedlite YN460 which is a tilt and swivel flash, from about £44, but delivery is from Hong Kong and you probably have to pay an import duty on top of the price and postage. It also takes 15 days to get to the uK.
Regards,
Denise