Vauxhall Signum diagnostic doesnt communicate with ecu

What diagnostics are you using?
and are you sure it's 04?
Yes 04 on the plate but when was the actual manufacturing date?
Diesels are only obd compliant AFTER 04,if yours is an old stock late 03 registered in 04 then generics wont read it.
There is a tool for checking the socket.Hinkley's sell it I think,around £80-100.
Doing it another way would need a 'scope.
If you have a laptop then cheap chinese copys of OPCOM will read it,if they work.
or try local ads for someone equiped with better scanners,my local autospark can usually pull code from anything.

Assuming you are using a good code reader you will have no trouble reading this model if you can't comms with the ECU you need to check the fuses first just in case one of the management fuses has blown, if there ok check the 16 pin plug for all good connections to code reader, if useing a cheap one, take to your local garage for a proffessional read

I have an 2004 3.0 Signum Auto which splutters when you try to acclerate or overtake, i have been told it could be a fuel pressure sensor