Ok,got it fixed today.Started first time after I fitted the complete ignition barrel.
I had assumed that the NATS system was similar to other cars.ie.Change ECU,ignition shroud & use donor key.
Not so,you need ECU,ignition barrel & associated key & the control module from behind heater motor.
If the NATS has kicked in on your car by far the easiest route to get it running would be to get a modified ECU from Ed on here.By the time you have trawled around in breakers yards looking for a secondhand system,stripped the donor,get robbed by the scrap man,dismantled your car,dash out & door cards off (if you want to keep your car on 1 key you'll need to change door/tailgate lock)and knock the security bolts off the column to change the ignition switch.After all that there's no guarantee how long the secondhand system you've fitted will last so you may have to go through it all again.I'm not too bothered as I'm going to save a spare system from one of our scrappers & I can fit complete in 10 mins as my car is stripped out but when I've finished my mods I want to get it mapped so may as well lose the unreliable immobiliser at the same time.
Still wonder if mods are causing problems with the system,ECU was designed for economical steady driving by nuns,not to have us doing everything humanly possible to milk every bit of power out of the car then thrashing the nuts off it...Went on some test runs today & had the management light flickering after ragging it hard.Must get a rev counter or put a distributor with a limiter fitted until I do...
Anyway,car works again so happy days