Is it an early 300 TDi .If so it may have an "anti theft" device .Located behind and above the front passenger footwell lhs plastic cover.The RAVE CD shows it. It is a black box about 6 inch long .If memory serves i shorted two black wire that go to pins 1 and 2 .I;ll try to find it on the RAVE and post later.
RAVE also suggests it may be the inhibiter switch that has gone open cct .try cleaning the plug .its on the left hand side of the gearbox .
If its a later 300 TDi they have a spider an immobiliser in the dash bd .There is a kit that can be bought called a "Spider" that overcomes a few issues i think ,but as mine has the anti theft have never looked at.I have seen a thread on landy zone though with pictures i think