The key lay in the back plate of one of my head torches, which happened to have several hooks and loops that you could pass a zip tie through.
Step 1: Materials Needed
2. One or more Zip ties
3. A head torch with the right kind of back plate*
*My petzl was not suitable, what worked was a head torch I picked up in the local supermarket. They no longer stock them but when I wanted to make a helmet torch for my wife I found numerous cheap lights online which use exactly the same moulding for the back plate. In the end I got one like that pictured above from ebay for under £2 including postage if you search "head torch watt" you should find something similar.
Step 2: Fixing the Light to the Helmet
On my helmet this could hardly have been easier all it took was a single zip tie.Due to the design of my wife's helmet it was a little more complicated but still far from difficult.
You'll just have to see what works with your helmet.
Whilst I was at it I thought I'd also fit a rear light, in each case this was incredibly easy requiring only a single zip tie, most rear lights I've seen would be suitable.
Step 3: Job Done
The lights make locking and unlocking the bike in our shed much easier and importantly also make us more visible to drivers, particularly at junctions where your handlebar mounted lights can be obscured by cars waiting next to you.