Righto chaps and chapettes, interesting developments!
I tried a tap thingy at various openings in the pressure line to the ECV, which I had plumbed up to line 12, as it was the closest line from the manifold to the ECV.
Fail, made no change to anything apparently, so ripped that out.
Then re-read the instructions on Subyclub, and he had it plumbed into line 22 from the secondary BOV/ SPV, derp!
So I've stuck it in that one instead and hey presto, it works as per normal, as far as I can tell from a drive round the block dodging the potholes down by the river.
Maybe a bit quicker to close the ECV after the usual weirdness from changing during VOD.
Will see if it does anything particularly noticeable next time I get to take it for a blat somewhere that isn't a pothole infested 50k zone...