Fitting the adjustable power unit was very easy and took only 20 minutes (including making a bracket!).
My unit was supplied by Dragon Performance (www.mrhoot.com) at a cost of £299 in April 2004. My car is a 2004 A170 which produces 95BHP and 180Nm of torque as supplied. The unit is supposed to improve the BHP figure to 120 and increase torque to 235Nm. It isn't listed on their website yet but email "Ade" and he will no doubt be as helpful as he was with me.

1. Find a location for the unit which is protected from direct water/roadspray/heat. I made up a mounting bracket from two 25mm hoseclips, but good quality cable ties or velcro straps would do. I mounted my unit on an unused earth bolt on the bulkhead. You can see from the picture that it is a good idea to keep the adjustment screw fairly accessible. To make things easy, I connected the supplied mini wiring harness prior to mounting the unit.

2. Locate the electrical connector plug on the end of the engine fuel rail. Mine had a yellow dot, and was obvious and easily reached. Just follow the clear diesel pipe down from the fuel filter and you'll get to it. Disconnect the plug and connect it to the input socket on the harness from the new unit.

3. Connect the output plug from the unit harness to the socket vacated by the original plug.

4. Tidy things up with a few cable ties. Nothing should be rubbing/vibrating with the engine running.

5. Finished, now time for testing !

I can only relate my own experience here. The unit will supply performance from the sublime to the ridiculous. The best way is to try it first, then adjust the unit up half of one division then try again. Each time the performance will increase. The technician I spoke to suggested upping the power until the engine management warning light comes on, (overfuelling alert) then back half of one division. He assured me it wouldn't log a fault on the Assyst computer.

The unit is supplied set at 60% which they calculated as 120BHP with a standard clean air filter. I found that when set to 90% the performance was too much for my style of driving, with massive amounts of power coming in hard at 2000rpm. I spent most of that test run on the brakes as the corners hurtled towards me ! I also noticed some fuel smoke under acceleration. I can only guess the power at this level was a long way over 120BHP, as overtaking was breathtaking (well, it sure was for the folk left behind in the diesel smoke !).
I found that the higher the performance, the noisier the engine became (combustion noise, bit like a poor exhaust).
After some experimenting, I have returned to the supplied 60% setting as I don't want to ruin my new engine or auto transmission, and I like to enjoy the passing scenery without the perception that I'm trying to get airborn from the deck of an aircraft carrier. At 60% the power is more subtle and the 2000rpm "kick" has turned into a more reasonable "shove".
Based on supplied figures from the unit manufacturer, and with the K&N air filter I have added, I am getting just over 120BHP, probably about 124BHP. I may put it on a rolling road later on, but intend for now to do some fuel consumption figures over the coming weeks. I have changed to a fully synthetic engine oil as well

Thanks to Graeme for the pictures and article