I am torn between these three cameras and wondered what people thought.
I've been offered the Canon 20D from a friend for £375 including a 20-85mm lens. But don't know whether to go for this or the canon 400D or sony a100.
I like the canon 400D but am bit worried about the lens. I can't afford too much really so was wondering if there was a relatively cheap lens i could buy to go with it that would do the job for travelling and taking pictures. Or should i go for the sony a100?
I don't know what kind of photographer you are, but if you're at all serious about wanting to do photography, and unless the 20D has taken more than 100.000 photos, definitely go for the 20D. Build quality and complete control make that a near-pro camera, where the other two are just lightweight consumer models. (And if a cheapie lightweight consumer model is what you'd really prefer, then I'd recommend the Nikon D40 over those other two choices instead.)
I don't know anything about the lens, so I'm just ignoring that completely in my recommendation here. But I figure that someone who owned a 20D won't have used a crappy lens (like the 18-55) on it, and that someone who knows Canon lenses will come in in a bit and tell you that it's a great lens, so that'd make the choice even easier.
I'm a beginner dslr user so don't want anything too complicated but what the potential to do more stuff as i get more competent using the camera
Well, it's still got all your basic "landscape"/"portrait"/"sports"/"nighttime" auto-modes, so you should be able to start out using it as little more than a point and shoot.
What I'd do is ask your friend to borrow the camera for a day, and go out shooting. See how it behaves, if its weight and heft makes you happy or wish for a lighter camera. Experiment a bit with different settings for the white-balance and auto-focus such - notice where the buttons for these things are, and then try to change them blindly (keep looking through the viewfinder, see if your fingers can do these things on their own). These are the kind of things that'll make the difference in use over the cheaper cameras.
couldn't find a 20-85mm but there is a 24-85mm USM
They gave it a good review except on the widest angle side. That's not unexpected. Most zooms get poor reviews on the wideside (To the point you get almost sick of hearing it.) It's got a good focal length range for most of your use. Hope that helps.
if you are a beginner, i would go for Canon 400D instead of 20D.
20D is definately a nicer camera. but definately not a beginner camera. (unless you got a really good deal on the 20D, personally i dont' know how much the 400D goes for in your area)
I would personally stay away from Sony.
Depending on what you want from a camera on this one.
The 400D, although is a newer model is built on a budget and it 'feels' it. I'm not saying it's not a great camera because it is.
As a starter camera it's fantastic, but if your wanting to a camera to 'grow' into I'd punch for the 20D.Mind you I'm bias as I bought one.
I'd stay away from the Sony simply on the basis that lens range is more limited than the Canon.
I would disagree with the previous two posts on the Sony A100. It's basically a Minolta lens mount. Which means you have access to a large selection of OEM and second party sources. It also has image stabilization built in to the body, which neither Canon or Nikon has. It's good value for the money.
Quoting Q'
I would disagree with the previous two posts on the Sony A100. It's basically a Minolta lens mount. Which means you have access to a large selection of OEM and second party sources. It also has image stabilization built in to the body, which neither Canon or Nikon has. It's good value for the money.
Agree! I bought this camera with a standard 18-55 mm lens mainly because I have two other Minolta lenses, one of 28-200 and one of 70-300 mm. Well, so far my response on the limited lens range which was said before. 
I own the Sony and am really happy with it! 
That said, Canon 400D wasn't out yet when I bought the sony and I probably would have gone with that one at that time if I didn't already have a few Minolta lenses. In Norway at least the Canon 400D is quite a bit cheaper than the Sony and the Nikon and would have been quite sufficient for my needs. But both the Sony and the Nikon have some features that the Canon doesn't and since it's Sony's first big push into dslr's I feel they've put more into it than the others did into similar models. Image stabilization is a BIG plus for most of us beginners
If the Canon20D is cheaper than both I would probably recommend that one, but like Sander says, try it out and see how it lies in the hand etc. (Sony won me over with that one as well).
Anyway, just thought I'd add my 2p.