"Another one to consider is the Sigma 50-150 f/2.8."...... That looks like a perfect lens for Angelhales's people, events and portraiture. Might even be considered a "good wedding camera". I must check that one out myself.
 Just checked the Sigma UK site - it's 715 pounds - not at all cheap, but something I may just try to save for [/edit]
Sigma's list price is seldom what the lenses sell for. It's £567 at Warehouse Express.
If you've got up to £500 to spend then the best long lens you'll get for your money is the Canon EF 70-300mm f/4-5.6 IS USM.
Of course, that requires you to have a Canon camera, which I think you do?
There are other options for these lenses, such as the Sigma ones and the Canon 55-250mm, but this one is pretty much the best one for Canon cameras that costs less than a grand.
That Canon on Warehouse Express looks brilliant! Out of interest what would that perform like in low light? Last night I've been invited to assist a photographer at a black tie ball to take informal shots etc. Would that perform well or not in low light?
I have a Canon 450D - that lense is compatible isnt it? I know theres something about full frame and stuff?!
the technical side of photography really escapes me sometimes lol
That lens is compatible with your 450D but in a darkened room the combo won't work too well without flash. You'll likely be better off with a fast prime, or at the least an f/2.8 standard zoom. If you don't have much of a budget for this then a 50mm f/1.8 would do a reasonable job for around £80 (it'll cope with shutter speed, but at times may struggle to autofocus. so be prepared to switch to manual focus occasionally).
I can't imagine that the 70-300 would be useful at a black tie ball, either from its modest aperture or for its focal length range. Your 50mm would do better, I should think. A shorter faster zoom (24-70 f2.8 or 50-150 f2.8) might be useful.
Why not ask the 'tog that you're assisting? They should know what to expect in terms of lighting, and what would be useful in terms of lenses.