I'll have a look at the carbon canister tomorrow. I know adjusting the cable will mask the problem, but at least it is now drivable! I have a pair of ISVs soaking in petrol overnight, I will try them both tomorrow as I'm pretty sure it is an ISV problem. Once the car was up to temp it would idle fine, and pulling the breather hose off from the ISV to the airbox and blocking it would starve the engine, which the other two wouldn't do. I guess they are both stuck closed whereas this one is a little bit open but stuck.
TB is clean as a whistle and all the breather pipes have been checked for splits and blockages and all the vac lines have Been replaced with 3mm ID silicone hoses.