" Check electrical operation of 3 way relay whilst engine on cold start : No vac' passes from relay to control valve, only boost pressure when opening throttle. Test light shows power to relay and most of the time to ground side but no vac' passes to control valve. "

What is the history of the car? Is the problem new, after recent repairs, hanging on for years or is this car new to the owner?

My somewhat similar 97 SL320 ( M 104 6 cyl ) runs the air pump for a few minutes at cold start and sometimes again when stopped after driving ~ 3 miles.

If I'm reading this right, the vac solenoid isn't letting vac to the air pump valve so no air is being pumped in to the exhaust.

If the front O2 sensor is lazy it might cause this code, do you have a spare to swap in?

As a test, I'd be real tempted to bypass the vac sol and allow operation when the air pump is running. ( be sure that constant operation won't allow exhaust to enter the air pump, if it does, jumper the vac sol to the air pump electrics. )

Also have a look at CrossfireForum.org as they might be of some help.

Make sure to post the results here for others.