In reply to Vaughany89:
The most important thing in my view is to get one that fits.
At the budget end, they're all going to be made of alloy, nothing wrong with that, just not the comfort and lightness of a £1,000 plus carbon bike...but you're unlikely to be doing the sort of rides where it really makes much difference (i.e. 100+ mile).
Groupset doesn't really matter, try to avoid sora, but the budget versions of SRAM and Campag seem fine in my view. tbh whatever you get, even sora, will do, if you like the sport, in 12 months you'll probably want to upgrade and have a clearer idea of what you want.
I'm always dubious about 2nd hand, there are perfectly good bikes new for £300 or so, I've seen many 2nd hand bike on ebay go for more than you would pay new for a similar spec bike. Of course you CAN get bargains, luck of the draw unless you know what you're looking for.