I've got one- a 2005 Trojan.

The usual cause of lack of power, assuming all filters clear and no fault lights etc is turbo overboost.

The symptoms :-

Put foot on throttle in high gear - engine starts to make power then fades out.

What's happening:-

The turbo starts to boost, the manifold pressure sender senses the inlet manifold pressure and sends the signal to the ECU which measures the pressure as excessive ie outside of it's acceptable map, so the ECU reduces the fuelling to protect the engine so the power fades out (it's a fly-by-wire-motor, mechanical distributor injection pump regulated electronically via an ECU and sensors).

The remedy:-

The turbo is a variable vane type-the vanes are operated by a vacumn servo which operates an external lever on the side of the turbo which has two adjustable stops.

The upper stop is the max boost scew, to reduce the boost, loosen the locknut and screw the stop in about one complete turn at a time, lock the locknut up and road test it. Repeat this until the power returns.

I had to move mine about 4-5 turns in to get it to power up properly.

The second thing to do is to join the L200 owner's club here -

www.l200.org.uk/community/

for loads of technical advice and assistance.

Hope this helps!