Demon Tweeks catalouge came through my door the other day and there was a big selection of good looking cheap helmets here >
http://www.demon-tweeks.co.uk/mx-off-road/helmets.
However as mentioned MX helmets are the series of some debate, the general consensus is that they are designed for high speed crashes and that MTB helmets are softer, more likely to break in a crash and thus asorb the shock and the shock on your brain is reduced and should be the preffered option.
I got a THE 'Empire' 2nd hand from Ebay which Im very pleased with. Its worth trying on some as they need to be tight fitting (and posibly you need smaller than you think).
http://www.chainreactioncycles.com/Mode ... elID=29457 (Unavailable
)
Andy