Is it possible to fry the ecu if you diconnect an ignition coil while the engine is running ,to see which cylinder isn't fireing if you know it is only running on 5cyl's?

 

I know that is a way to fry the EFI computer on my Rover. For your sake I hope that is not the case on a Jaguar.

 

could be, but I would bet you did not fry it. Don't panic if you did- you can get one much cheaper than the dealer's charge.
Why do you think it is dead? Do you have a 12 volt test light? Multimeter? Access to a Scope? (just though I'd ask that last one!)