Just purchased the 7 led - halogen main headset with long cable and battery box which accepts the flat 4.5v batteries. Initial reaction is very good. Will report after some bigger trips.

So I'm now thinking that if the 7 led's can run off a FX3 battery 3.6v then the flatpack 4.5v isn't absolutely necessary and I could use 3*1.2v AA = 3.6v! Yes?

Indeed. I've run mine (14 LED) off rechargable AA's and it works fine. I also know people run them off very cheap (Tesco value!) disposable AA's too, also without any problems.
The main advantages are - availability, and the very light weight.

Just been talking to my brother who is a sparky at Lancashire CC. He got interested in this whole lamp business when I talked to him about a while ago. He has purchased from the internet some oscillitaing relay widget which has allowed him to run 3 leds from a 1.2v AA battery for 20 hours! He was quite impressed with this as was I. Off to visit him this weekend with headset and shopping list.

where did you purchase said item (out of interest)

The headset? Inglesport. I'm sure they said it was some bod from Speleo who had done the mod.

The widget? Will ask.