My 1996 Defender with a 300 Tdi has recently awaken form a long coma...
It starts fine, and idles perfect. The moment i depress the accellerator the engine loses power and dies.

Any thougts on the reason and possible cure??

Check the basics first

Air filter, fuel filter

Then try running off a separate fuel supply direct to the fuel pump

Fuel starvation or air starvation.

If after it has started and ticked over for a minute ot two, when you press the throttle is there black smoke from the exhaust ... air filter blocked - remove the air filter and try again.
After ticking over does the engine simply die when the throttle is pressed (no smoke) - fuel starvation.
Check/replace the fuel filter.
Check the diaphram on the top of the fuel pump (FiP) - can you push it down against the spring with your finger ?

If all tests pass, use a separate fuel supply direct to the FiP