I used Citrojim's excellent guide below:
http://www.frenchcarforum.co.uk/forum/v ... hp?t=33767" onclick="window.open(this.href);return false;
Rather than following the guide step for step I decided to do the easiest things first and work my way towards the more difficult, because I'm fundamentally a lazy person.

For me the easiest thing to do first was electrical tests from the ECU connector, but finding it proved to be the first challenge. According to Jim's guide the ECU is under the carpet under the drivers seat... not on my car it isn't !!

After considerable head scratching and going back and forth between Jim's guide and the vague location shown on the Sedre wiring diagrams I realised that it's in fact inside the centre console behind the rear electric window buttons, so is easily accessed by removing the tray behind the window buttons and the buttons themselves.

Wiring diagrams for my car, Series 2 RPO 7905 taken from Sedre:
https://dl.dropboxusercontent.com/u/782 ... nciple.pdf" onclick="window.open(this.href);return false;
https://dl.dropboxusercontent.com/u/782 ... lation.pdf" onclick="window.open(this.href);return false;
https://dl.dropboxusercontent.com/u/782 ... Wiring.pdf" onclick="window.open(this.href);return false;
I diverged somewhat from Jim's testing procedure, the first thing I tested was the power feed between pins 10 and 8 with the Cruise switch enabled - check.
Next I tested for power between pins 9 and 8, which should be present with cruise control turned on and the brake pedal NOT pressed - fail. This comes through relay 7307 and switch 2100 which is in the brake pedal. I checked on pin 1 on the brake pedal connector and had 12 volts, on pin 3 nothing with the pedal pressed or not pressed, so as suggested in Jim's guide the brake pedal contacts were faulty.
I pulled the brake pedal switch apart and cleaned the dirty contact with emery paper as recommended and the brake switch is now working properly. Reassembled and plugged back in, we now have 12 volts on pin 3 of the brake pedal switch when its NOT pressed, as it should be. One fault fixed.
I then returned to pin 9 of the ECU - still nothing. Hmm. Relay 7307's coil is powered by the same cruise switch that supplies power to pin 10 of the ECU so I know that switch is ok, I know the voltage out of the brake pedal switch is now ok, so either the relay or the wiring to it is faulty.
Now the fun begins, because I'm not actually sure where it is! Jim's guide says in S1 cars its above the right hand wheel arch liner, and S2 cars have it in the drivers foot well relay area. Mine is a S2 car, yet the diagrams from Sedre with the correct RPO number entered (linked above) clearly show relay 7307 mounted above the wheel arch liner.

Further confusion surrounds pin 6 and 7 on the ECU - according to Jim's guide the function of these two is opposite in S1 and S2 cars - in S1 pin 6 is electrovalve and pin 7 is motor, in S2 pin 6 is motor and pin 7 is electrovalve. (Trust Citroen to do something like this!)
Well in my car pin 6 is definitely the electrovalve, and pin 7 is the vacuum pump, consistent with S1 wiring despite being a S2 car, yet the wiring diagram from Sedre for the correct RPO says that mine should be wired opposite, eg in S2 fashion!

It's a bit of a pain to take the wheel arch cover out and I don't have a spare relay so I decided to leave the relay investigation until tomorrow - I'll go and buy a relay and have one on hand when I take the wheel arch liner out so I'm not having to do that twice...
I then turned my attention to the pump, dump valve and diaphragm. Not having a small battery to connect to the ECU socket as Jim's guide suggests, I used power from pin 10 of the socket itself.
With pins 6, 7 and 8 bridged together (earthing the earth returns of the vacuum pump motor and its solenoid) and pins 10 and 1 bridged together via about 3 metres of cable, I was able to stand and listen under the bonnet while I connected and disconnected the power, and confirmed that the vacuum pump is working. It's kinda hard to miss really, its rather loud!!

If I put my finger on the diaphragm I could feel it pulsating but there wasn't enough vacuum to activate it. On a previous occasion I've checked all the vacuum hoses and they're all in good condition, on the right spigots, and all nice and tight, so I don't think its a vacuum leak with any of the hoses.
To check the diaphragm was ok I disconnected the hose from it, opened the throttle wide, put my finger on the hole and it holds the throttle open without any leakage, so no vacuum leaks in the diaphragm itself.

Next I checked the vacuum pump and its electrovalve separately - which is how I found pin 6 and 7 are labelled incorrectly on the diagram as pin 6 is definitely the electrovalve and 7 the motor itself. There is a distinct click from the electrovalve, so I'm assuming for now its ok. The electrovalve draws about 170mA, and the motor itself about 400mA.
Next I disconnected pins 6, 7, 8 and just supplied power from pin 10 to pin 1, which should operate the separate dump valve, as its earthed through the chassis. Nada, nothing. With my long wires I was able to lie under the corner of the car and listen while pulsing it on and off and couldn't hear a thing, so it looks like the separate dump valve is not working.
It does seem to be drawing some current though - about 30mA which isn't really enough for an electrovalve, so I'm hoping its just a corroded connection. I've previously had the connector off the valve and sprayed it with contact cleaner, but I didn't realise at the time that the earth return goes via a chassis lug somewhere, so that could be where the fault is. Either that or the dump valve is faulty.
So that's where I'm at at the moment. So far I have found three individual faults - brake pedal switch, cruise control relay and dump valve, all faulty, no wonder it wasn't working!

I think the ECU is working ok though because if I'm driving above 25mph and I try to activate it a relay in the ECU box clicks, then clicks again if I press the brake pedal or pull the cancel direction on the stalk, so at the very least its sensing speed and inputs correctly. I've also ohmed the control stalk to check the switches - at first they were very "scratchy" probably because they never get used, but after a few minutes of working the stalk in every direction the contacts now seem fairly reliable. I've run out of contact cleaner so I can't spray the switch just now.
Hopefully tomorrow I can find where this relay is and fix/replace it leaving just the dump valve, which I'm not looking forward to as access is terrible especially when you're working on the pavement!

If the relay is in the drivers foot well area I have no idea which one it might be since the wiring diagram doesn't show it as being there...there are also two empty relay sockets in the foot well area, not sure if that's normal!