Fuel light

I run a 1992 Defender 90 with a 200tdi engine. Normally the fuel light comes on when I first turn the key and goes out after the engine fires. It also lights when the fuel gauge drops to the red sector as you'd expect. Yesterday I was driving home from work and the fuel light flickered a few times before staying lit. Today it stayed lit for the whole time I was driving. The gauge appears to be working normally. Any ideas what could be causing this? The engine has been a bit lumpy when first starting for the last couple of (very cold) days - could this perhaps indicate a glow plug problem?

The gauge works using 2 wires, (there are 3 but 1 doesn't actually do anything).

When the fuel is low enough the light will illuminate; when the fuel is low there is a high resistance - less current - needle only moves up a little... this also triggers the light circuit.

The light could come on because the system described above has a fault or there is a stray current driving it, or an earth fault. Take up the drivers seat and acess panel and pull the plug off. If the lights stays on it's probably a stray current.

Lumpy starting on COLD day could indeed be the plugs. Meter them out and see if they are open circuit.