The Carrera Radius helmet is the top-of-the-range model from specialist manufacturer Carrera. It's pretty light, claims good aero credentials, and has an adjustable set of straps at the back to hold it firmly on your head.
I have a nearly new LARGE Carrera Vanquish for sale (I cant afford to run 3 bikes).
Its covered less than 200 miles from new, had its first service and runs like a dream.
Ideal winter or commuter bike as will carry panniers easily.
Cash on collection. I work in Bristol and Basingstoke so can make the bike available in either location or anywhere in between with notice.
The new model is on sale for £550 but with a lower spec (comes without the carbon forks).
£300 ono so grab yourself a bargain.