Well one thing I would test is the wire to the fuel shutoff (back of pump 1 wire on its own) see if it has power all the time or when key is off then it should not have any power there that would be why it won't stop on the key or bits inside been taken out
Just a idea but if you run a wire from the bat to the fuel shutoff on the back of the pump and see if it starts then it will rule that out as you may have a broken wire on that
I had the same thing happen on my 300 tdi and it turn'd out to be the little leak off pipe had split but I don't know to much about td5 but may give you some were to start
I did a engine change on my friends disco (has a 2in lift kit) I never would do one with out dropping the gearbox out of the way 1st so I would get a transmission jack and do it on ramps
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