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Head over to somewhere like Motoden or Metropolis (don't buy from Metropolis) and try lots of different manufacturer fits.
Once you know which fit works for your head shape (i have a rounder face and Shoei fit great), buy the one you want, online, for cheaper.
Features i would have as a minimum:
- Good ventilation
- Removable ear cups
- Resizeable cheek pads
A helmet is one of the main things it's worth spending more on (along with gloves and boots). The more you spend, the quieter, lighter and more comfortable they will be.
- Good ventilation
If you know your fit, you can't beat http://www.sportsbikeshop.co.uk for service and value.
They have some decent items on discount at the moment...