HI,
can anyone give me an objective review of the 1GB Radeon HD 4850, or offer an alternative that is better at a similar price (£84).

Personally I'd get a 5750 as it's a little bit cheaper but supports DX11, HD audio over HDMI & is offers similar (if not faster) performance

For just a tiny bit more money than the 5750 the 5770 runs circles around it. If you can squeeze to that I dont think you'd regret it (although I understand its more than the £84 you originally quoted).

How much more?

They're about £120

are they that much better?

The 5770's score about 1000 points more in 3DM06 & 6-10 fps more in games.

Depends if you want to spend the £50 extra & how big your screen is.

My screen at the mo is not that big, can't remember the exact size, I will be gettign a larger one in the near future....

For a 22" or above a 5770 would be better.
All depends on what you play & how good you are at sticking to your budget (as every model up is only £30 away)

£113 for the 512MB model (which might be all you need if you dont game at big resolutions)

Clicky

This is a review of two cards, same brand, one 5750 and one 5770. There's up to 10fps difference in the Batman Arkham test at different resolution until you get to the really big resolutions when the 5750 cant realistically run the game.

My Current monitor is 17" and I don't think I would go to a 22" as it would be too big for where I have it. 19" would probably be as large at the mo...

You probably don't need a fully top end card then. I have a 19" 1440*900 monitor (resolution is more important here than size) and a measly HD 4650 takes care of all my Modern Warfare 2 / Fallout 3 needs at full settings. It won't do Crysis properly though, so as mentioned it all depends what you're aiming for in terms of games.

Unless your monitor is going to be 1200*1600 or more, most recommendations are going to be a little higher than you need (assuming Crysis is not on your wish list). I'd say a HD 5750 (starting about £102 on eBuyer for 1GB, 512MB only £89) would cover pretty much everything, including the planned monitor upgrade. I believe that the 4850 is now getting hard to find these days, so watch you don't pay too much for it.

Will it run 2 monitors? I do this at work and it would be helpful at home too...

Yes. The HD 5750 even supports eyefinity across 3 monitors at once, should you be crazy enough to try.