Hi Steve,
Welcome to the addiction, I see we have another victim
Since youve already got a CX Im assuming you can hover and fly around a bit.. Where you go next depends greatly on how much space you have to fly it. If your a club member, then Id suggest speaking with them and then getting what is most common.
In principle Id say your next step would be a trex450 size collective pitch machine. Unfortunately, to get one of those, you'll need to revise your budget up a chunk. I know it is possible to get CP machines such as walkeras for the money your talking about but I would advise you to steer clear of the very cheap machines because the electronics can be poor (of course you might be lucky).
For 300 quid you can get a blade 400 3D which is a great machine for the money ..
http://www.modelhelicopters.co.uk/ac..._Electric.html
In the above link you'll also see a machine that would probably be good for you at 259 quid (I-GL450SEC).
Hope this helps.... Azzy

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How much?
JP Bell 47 Twister
Align T-Rex 450 XL (well it was once)
Align T-Rex Airwolf
Align T-Rex 600N Sport
Align T-Rex 600N Pro
Align T-Rex 600e Jetranger
Comming Soon - Graupner AS350 TwinStar