Whilst doing all the leak hunting, I also noticed that the front multiram wasn't moving at all. It sits at the top of the front intake pipes, and is supposed to open above 3,400 RPM. I checked the vacuum valve itself with 12V on the bench and that was fine. The valve is fed vacuum from the reservoir tank at the back of the bagpipes, along the white hose you see. When the ECU opens the valve, the vacuum pulls the solenoid arm in, which then rotates the spring-loaded cam of the multiram valve and opens it. Except it didn't budge.
Lots of squirts of spray degreaser and manual moving of the cam arm soon freed it up though. I guess it dried up and the spring stiffened over the years. May be an idea to check yours - if you've got this far

Gave it a quick squirt of silicone grease and it's now merrily rocking back and forth when I tweak the throttle.
