Scaffolding was old stock from a local scaffolders (I wanted the distressed industrial look, just given 'em a ring).
Fittings are from ebay (there are standard sizes and joints), the legs are spinners (if you turn the handles the desk moves up and down).
The top panel is 28mm structural birch ply (structural ply may have surface blemishes and patches rather than finishing ply so if that matters get finishing ply but since I wanted the industrial look and it's way cheaper to boot), routed with a 5mm edge and covered in 4 coats of heat resistant wax (makes it water proof without ruining the color).
Original panel was 8ft x 4ft so I took a strip of the long side for the shelf (if you work out your dimensions most timber places will make the cuts for you).
Through holes are 60mm and there are 3 on the back (UK plugs are massive).
http://i.imgur.com/YzfWv5E.jpg in construction, two cross beams are for the shelf which sits on them, the middle upright is because even 28mm play will sag.
Total height range is ~85cm (I'm quite tall and normal desks are a tad too low) to ~140cm.
Would work nice on a smaller desk though :).
As for sturdiness, it's weight and rigidity make it a very stable desk.
I've already had two people ask me to make them one (with a nice margin on top), I think a lot of people are tired of crappy fall apart desks.