This week's special challenge pits the Anoraks, winners of the 2004 series, against the Catalysts - Gary, Tim, and Shane - winners of the 2001 series and of every Weld Cup Final since. Their challenge will take twenty hours to complete. It's a build-your-own juggernaut contest, capable of pulling four times its own weight in timber. It'll be judged on load-to-weight ratio, and there can only be one powered axle.

With the Anoraks is Mark Hale, a lorry engineer who specialises in custom-build trucks. They go for something powerful, geared, with lots of traction. They want a large flat bed, with ballast on the back, and will need to construct a towing hitch to connect. The ballast will determine whether the front is too high, too low, or just right; that and the towing hitch will be the key builds.

The Catalysts have Richard Vincent, who builds tractors and engines. They head for a basic tractor shape, with a much larger engine. They'll have no real traction problems, as their trouble will be finding the right engine, and making sure the towing hitch is strong enough.

Lorries are hard to miss, even on the heap, and the Anoraks find their truck, with a turning engine. Turning, but not going that well. For the Catalysts, their tractor has everything but the seat.

The Anoraks' lorry is well buried, and it's some hours before they can remove all the debris and free it for work. They also need to bypass the fuel system, and then bypass the door, and repair two flat tyres. Once the lorry finally arrives back home, they have to strip off as much weight from the lorry as possible: excess weight on the lorry could count against them.

The Catalysts have problems: they need to modify the oil system, as the original engine would take in an oil-air mixture. There's a difference of opinion on whether they need a six-inch-thick towing hitch or something more simple. The gearing on their engine seems to be stuck so that the lowest gear can't be selected. And they have no tyres. Not a happy warren of bunnies.

On day two, tyres are the requirement for both teams. The Anoraks get a lot of tyres, the Catalysts get almost none, and the Anoraks get first pick of the tyres. The Anoraks find a lot of ballast, twelve barrels giving almost 2.5 tonnes, and their build is in good shape.

Things go from bad to worse on the other side of the fence - expert Richard spends most of the morning tinkering with the engine, connects everything up - except the oil filter. Oil everywhere. The team have four workable tyres, and attach lengths of pipeline to the one drive axle so they fit. Weight comes from 250kg of water on the front, and solid tyres made from concrete. It's finished, but no-one seems to have much confidence in the machine.

Come testing day, the Anoraks will go first. They start with one tree trunk (1.3 tonnes), and both teams manage to pull without really breaking sweat. The second pull is four trunks, about 5.2 tonnes. The Anoraks make it, the Cats lose traction early, turn the engine up to max, only to see it rather expire under their feet. The Anoraks almost manage the third run, making 50m of the 60m course pulling fully six logs and 7.8 tonnes. Had the ground been harder, had there not been much rain in the previous weeks, they may well have made it. They've done more than enough to win.