The vacuum canister at the back of the plenum has a load of hoses coming off it.
The bottom 2 go into the plenum cast.
Those had been left open when the hoses had been removed.
They've been like it since I bought the car back and I hadn't noticed when I removed the plenum before.
I removed it today to swap the iac over and noticed them open.
So I removed the canister and checked all the hoses and then blew through it and noticed it was also blowing through those.
So I put it all back on and have looped the pipes that come out of the plenum cast and hey presto the car idles and drives perfect now
No wonder I couldn't make good boost with a vacuum leak as big as that.
Good job I'm no boost monster
Car goes in for paint in 2 weeks
I will have a running and looking great car when I get it back