An explaination of EF-s mount based on when the format was first introduced
http://www.bobatkins.com/photography...s/efs-10d.html
I had a Canon 28mm f2.8 and to be honest I was not that empressed with it for a prime lens. Heavy CA (Colour fringing) on the edge of the frame. I've had zoom lens which perform better than this beast.
It's field of view is equivalent to a normal lens 50mm using a full frame 35mm camera on the 400D. i.e not wide angle. 28mm x 1.6 = 44.5mm
A cheaper alternative to the Canon 10-22 is the Sigma 10-20 but your be lucky to get at sub £200.
Are you suffering with too much distortion with the 18-55 ? or is it not wide enough to get tall building into view ?
I see that ebay is currently showing some Canon 24mm EF F2.8 in this price range. Photozone review/test show better results than for the 28mm f2.8.
I would avoid secondhand old models of Sigma as they may not work correctly on newer Canon DSLR due to electronic incompatibilities.
Trying to find a sub £200 good quality wide angle is not going to be easy.