I have fitted a recondition 2.0litre injection engine,but since doing this both the red and green lights on the immobiliser key pad stay lit.
It is a 1996 model and it was my dads car and so have the original keys and correct codes.

Any Ideas Please!

The cables that connect the keypads are notorious for breaking, they are very thin and brittle, and the consistant flexing is enough to break them, resulting in problems.

You can get a repair loom which is better, as its more flexible. It is expensive, and costly to fit.

In your case, I would recommend that you try to remove the keypad, and check for any broken wires at the back.

Once you have the pad working again, start the engine, and then disconnect the loom from the back of the keypad WITH THE ENGINE RUNNING.

I disabled mine in this way when I had the 1997 Ulysse, my 2000 hasn't got a keypad.