 You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
I need a spade for a bit of trail fairy-ing... just digging drainage, not really constructing berms and jumps.
Are there any that are ok to use, not too big (I ride with a ~12 L camelbak) and not too heavy?
I'd go to the local army surplus place and look at entrenching tools.
APF
they are not very good for digging compact ground or anyting with roots. The ones with a mini pick might help. A mini mattock would work for drainage channels on compact ground but always a compromise compared to hiding a full size tool in the trees.
Son uses one of these - feels fairly strong, folds down nicely.
http://www.amazon.co.uk/Steel-Folding-Foldable-Camping-Shovel/dp/B002JTLKKY
Foldy spades generally aren't very good but they do the job. This is probably no use since I can't recommend a source but I have a small adze that I got in a random diy shop in London, I call it my pocket mattock, blade is about 3 inches, head has just enough weight in it, handle is no more than a foot and a half- excellent for cutting drains etc and it easily travels in or strapped to a camelbak.
I've got one of these:
http://www.screwfix.com/p/roughneck-round-point-micro-shovel-27/53829
Its plenty strong enough, small, light, and I managed to attach it to my backpack and ride up no problems.
Gerber do a nice one that is lighter than the ex service ones, as well a being smaller. It is ideal for small clearing/digging, but not much more.
http://www.amazon.co.uk/Gerber-Gorge-Folding-Shovel-Black/dp/B000WZCSTO