Glad to hear about the outcome. The less common causes are usually the hardest to figure out.

Problem was partially severed throttle cable which was catching and causing the butterfly to stay open, but didn't show up when I checked the butterfly!
How annoying.

Sounds like intake piping is wrong, or aac valve is stuck open. first make sure that the only hose that bypasses the throttlebody is the one that feeds the pipe that goes to the AAC Idle control block.

Engine is a DET. Yes I did pull the intercooler hose off to check the throttle was closing.
Must be a leak somewhere else. I remember it happened while I was coming out of a corner on boost.

Would be a huge leak though as it reaches redline in 2 seconds.

have you pulled the air hose of the front of the inlet to check the throttle is closing

iff the idle screw is set with idle too high and there is no load on motor then yeah it will just rev up,

got to be throttle not shut

Is it a DET or just a DE?

ECU cannot cause the engine to rev over redline -- so why are you replacing it? The ECU can only limit the redline -- it can't exceed it.

If your throttles are actually closed, air is getting to the engine elsewhere.

It is an aftermarket ecu, so I'll replace that. My feeling was that it would be an air rather than fuel leak, so perhaps the intake manifold gasket has blown a hole or as you say the IAC is stuck.
Thanks

Heard of electrical issues with aftermarket installs where they remove rev limiter and as older ecu had no cold start some have issues due to faulty sensors

You must have a massive air leak. It doesn't matter about the ECU -- the ECU can't supply air to the engine.

Maybe the IAC is stuck open or something.