Hi!
A time ago I asked for advice becaues my wifes Defender 90 2.5TD hat problems cranking in the cold winter mornings.
It was not the battery, testing it showed no weakness. But when checking the glow plugs, I found at the second one that it was totally broken (although Champion brand and only one year old, seems that they applied to much torque when connecting the wires!), even the connector bolt was loose. So we went for new ones, and now everything is fine again; 45 seconds of heating, and the engine starts up at first try.
Btw., it seems to need abt. 45 seconds; with 30 it took two tries.
regards - Ralph