£350 to Spend

For 350 quid I would suggest that you look at the second-hand market to try and maximise what you can get for the money. Cycling Plus recently had a round-up of entry-level bikes and they were looking at higher prices than £350. The main problem is that at that price almost everything on the bike will be a compromise of some sort, leading to lowered performance and higher maintenance costs in the future. At a slightly higher price point, you can look at getting a bike with a good frame at least, but with lower-grade components. The cheap framed bike will require a complete replacement to upgrade, the bike with a good frame can be upgradede a component at a time.

My recommendation therefore would be to scan sites for cycle clubs around your area and see if any of the more serious riders are selling off their older kit. That way you should be able to get your hands on a bike that has a good aluminium frame and moderate components with a value of perhaps £500 if it were new.

I did something very similar myself this year. I had slightly more budget than you and managed to get my hands on a 3 month old Batavus Pro that a local pro rider had been given by his sponsor - he had then changed sponsor and was abel to sell off the bike. I picked up a £1000 bike for around £650.