As a result of reading all of this thread and everything else I can find, I have bought an Alpha helmet. The clinching act was sitting in the cockpit with the lap belt tight and leaning forward to see which bit would hit the cyclic. Answer, the forehead! That coolie hat trimmer on the top looks very unpleasant to head butt.
There are a constant stream of ex military helmets going through Ebay. Expect to pay around £400 plus the cost of a mic and adapting the avionics for civilian use (not difficult if you can solder). Civilian helmets come up less often and seem cheaper, probably because the collectors are not interested. They are more likely to come with the right impedences and a mic. I got a non military one.
Anyone else decided to go hard hat as a result of reading this?
As with everything on Ebay, Buyer beware.