most likely you CO is at fault
On my 2wd i used to run my CO higher than the MOT limit so it ticked over ok
I take it they lowered the Co reading to pass the MOT

With the idle valve plugged in turn the screw on the ecu clockwise half a turn at a time till the tickover stabilizes...
im assuming you cossie is not standard hence the larger injectors give u a rich CO at idle

Having said all that my escort goes through the mot on a P8 and greys