how do i bypass the immobilizer, as the diesel pump is off a later car?

I have a 'p' reg 306 19td, and the lucas fuel pump packed up.
After speaking to several so called 'mechanics' all with different versions of an answer, they said, as long as the replacement pump was still a lucas..then i could bypass the immobiliser, and just fit another solenoid.
we've changed the pump for another lucas, out of a 'T' reg car, and tried to bypass the immobiliser, but it still won't start.
When the solenoids removed, and the car is turned over...fuel pumps out from where the solenoid screws in, but when the solenoids replaced, and a live bypass cable is attached...the fuel still doesn't reach the fuel lines going to the engine...?
what stops the fuel, passing through the solenoid, and getting to the pipes...?//