Have I killed my ECU

[horndog]

If its backfiring then it's got some spark. Are all plugs firing when you turn the motor over. What do you mean by decompressed anyway? Did you put a decompression plate under the barrels?
Is the fuel pump working? Are all the injectors working (you'll feel them click when you turn the motor over).

Those ECUs are pretty tough. I doubt if a quick spark from the dash would have killed it. But then....

Camel

Yes spacer plate under the barrell.
It seems to be intermittently sparking which could lend itself to a bad earth but it is cranking over ok. Pumps working ok but I still need to check the injectors.