We have fitted a new engine in my x type it does start but only revs to 2500 the engine sounds good ,I have sent it for diagnostic checks now it won't run at all I have changed the ecu but it is saying that the imobaliser is not talking to the engine please help

What are the results of the diagnostics? If you do not have the correct PCM computer it will not communicate with the anti theft. did you replace the '04 x type engine with the correct year PCM that's the cars computer it needs to match the cars year and be programed for the Jaguar x type 2004 car. This is all basic electronics and computer programing / flashing same as firmware where on any computer. If you don't know how, find a service center that has the Ford / Jaguar Scan tool IDS-VCM

Your first mistake was buying the 21st Century Jag- your second mistake was thinking you could fix it-

Jamnblues, from my brain to this forum. That's another 10 points for you.