Nice piece of lateral thinking. I hadn't thought of that so I started this evening by looking at this. I connected the manometer up again and disconnected the hose to the air filter completely. Unfortunately the readings were the same.

I've borrowed a compression tester but it won't fit a 2cv as the plugs are too recessed to tighten it up.

Finally, I dug the old breather out of my box of bits and swapped it with the new one. Quite amazing really, at idle it sucks the fluid right up the tube of the manometer (about 280mm) and it sucks nearly as well at high revs.
So that appears to be that. I spoke to the chaps at Ecas earlier and I'll have to be sending this back to them. Shame really as it looks brilliant quality and in theory it'll last forever of course.

Hey-ho.

Thanks very much to everybody for the assistance. It's good 'ere init?