read somewhere the new helmets have some type of Mesh lining to aide ventilation.
think it may have been in the MOD news paper with an article on the Clothing IPT & the new range of Dessie kit they have recently trialed/brought out.
The new helmet is the Mk6a, and is being rolled out to operational theatres as I type. It is a one for one exchange in the stores, so you keep it when you return back to the UK. I imagine that eventually everyone will be issued it.
It is a damn sight more comfortable than the original Mk6 with the annoying green string...