If I recall correctly, the sequence is:
Indicator>Altenator>Water Temperature>Oil Pressure>Main Beam.
Either way, the three in the middle are all red LEDs, and are all the same as far as I can tell. Indicator is a green LED and Main Beam is a white bulb with a blue cover.
If you take another of the red LEDs out, and take it to a local electrical shop/Maplin, they may be able to match you a similar replacement. Failing that, I have a couple lying around out of a spare (and dismantled!) set of MK2 instruments if you want me to post you one for a small fee.
Note that when replacing it, an LED is a diode, and will only work one way - if it doesn't seem to be working, it's in the wrong way round!