Read Paul's post with interest, and I have to admit that while I am all for re-cycling I think that trying to re-cycle the Petzel would not be cost effective. You can get a new Cree XML T6 for the same price, complete with adapters, so that you can use it as a head lamp, helmet lamp or cycle lamp. The power of these lamps is awsome and in all honesty most would use the lower setting. The down side is that it has a relatively short battery life and you would have to carry a spare battery on any night walk

Petzl Zoom upgrade.
Lots of people must have Petzl Zoom head torches lying around unused. They can be put to good use with a LED module from Customduo, (I don't know how to put in a link, but you should be able to find it with Google) called the Mooz. I had previously tried one of the drop-in LED bulbs, but it gave a horrible ringy beam which felt like wearing goggles. The Mooz is available in two versions, one with a single 130 lumens setting and one with three levels of up to 250 lumens, priced at £24 and £30 respectively. I went for the simpler version and used it on the Camel-Teign 100. I found it just about perfect. The nice wide beam of neutral white light let me get a feel for what I was walking through, rather than just giving the little pool of light some head torches give and the batteries (Cheap AA alkalines) lasted the whole night. The old flat 4.5v ones are reckoned to last 16hrs.
Fitting the module isn't quite as simple as dropping in a new bulb- you have to remove the old reflector- but doesn't neeed more than a screwdriver and pliers. Full instructions are on the website.
I'd recommend trying one of these if you've got a Zoom (If not they're quite cheap on ebay) and want to give it a new lease of life. It's still a very comfortable head torch to wear all night and is just about indestructible.
I have no connection with Customduo other than as a satisfied customer.

These ones from Alpkit are great head torches, equally as good as ones which are a lot more expensive. Unfortunately they seem to be out of stock at the moment.

http://www.alpkit.com/shop/cart.php?target=product&product_id=16345&category_id=25

There are some great Cree LED torches on ebay at a fraction of the price available elsewhere and with outstanding performance for under a tenner. Just used one on the Hadrians Wall Rat Race, really pleased with it. Try 600Lm CREE XP-G R5 LED Headlamp or similar with a variable strength beam for track & GPS/map. I spent £30 on a Petzl which turned out to be too weak to light the way properly.

Glad you found the review useful, Rob. Your Tikka Plus is ideal for RD reading, and you certainly don’t need a MYO. I got mine years ago, before I started experimenting with hand torches, so it makes sense to keep using it for RD reading. If I was starting afresh I wouldn’t get a headtorch that powerful.

Elton, thanks for the tip. I've just picked up the torch you recommended from Maplins. I wish I had read this before the 100. I really could have done with the extra light, especially on my second night when crossing Horsell & Cobham Common. I'm not sure my budget extends to the head torch at ~£65, but I may be able to manage with my old Petzl Tikka plus, now that I have the hand torch.

With the Games Hundred coming up, I though it might be helpful to share my torch experiences.

Usually I use a Petzl MYO XP head torch for night walk navigation. Set on the second brightness setting, which is adequate for lighting the path, the batteries last around 70 hours. It has a little plastic diffuser which flaps up to reduce light intensity for reading the route description. It takes 3 AA batteries and has a little indicator light showing when remaining battery power gets below 30%.

http://www.petzl.com/en/outdoor/headlamps/performance/myo-rxp

However, I prefer a handtorch for lighting the path. I’ve done a few night walks holding the head torch in my hand, but found it more difficult to handle than a hand torch.

On the Games Hundred Marshals’ Walk I used on of these.

http://www.maplin.co.uk/flare-led-torch-with-cree-technology-507167

It uses AA batteries, same as my Petzl and GPSr.
The beam can be changed from spot to flood (4x)
It is powerful: on spot one can read street name signs at 50 feet.
It has a range of brightnesses and hence (most important to us) power consumptions.
Full power: an AA 2450 mAh rechargeable battery lasted 2.25 hours. However, I found this level almost too bright for navigation. I found that 40% brightness was adequate. At this level, the single AA battery lasted over 4 hours, so 2 batteries will be adequate for a night.
With an AA battery, it weights 125 grams.

I find the path lighting setting too bright for reading the RD, and switching settings back and forth while on the move is not really an option, besides which switches tend to become non-functional before the bulb on these LED torches. So I still carry my Petzl for reading the RD, and backup.