what exactly is too high? most commonly it will be an exhaust leak. but could be a few other things like the o2 sensor or coolant temperature sensor. for the sake of £20, i would put a new o2 sensor in and then check for exhaust leaks and try again.
Can also be a leak in a vacuum hose (seen this twice in person so that's twice more than a faulty lambda lol)
Process of elimination sounds your best option mate. Keep us posted with progress.
A leak in the vacuum hose at the MAP sensor could cause it but if you've already checked for leaks that should rule it out.
You running stock injectors, stock fuel regulator and standard pump yeah?