The shocks on my year 2000 TD5 defender are on their way out and I'm looking at Konis as a replacement.
It's ex military and has uprated suspension with twin coil springs on the rear.

I would be grateful for any help as to which model to use - Raid, Heavy Track etc. Not sure what else I need to know. Are there different lengths available? How can I tell what spec is already fitted?

The vehicle is permanently loaded - probably to about 3 tons - and won't vary from this much.

Have a look on the OME website, they used to list details for spring and shock weights etc. OME