do I need to fit a bypass relay for trailer socket on '96 discovery?

Nope, they're only really needed on modern stuff that uses "can-bus" type wiring for bulb-fail indicators, and a Disco series 1 of any incarnation is pre can-bus. Badger.

This is still wired conventionally.

If you go here:

(This site seems to require IE or Firefox plus IEtab/IEtab2 to work properly.)

The official wiring loom (ANR3896) mentioned before is a very pricey way of doing this, although very easy. There's nothing to stop you cutting off the fitted socket and connecting your 12N socket as you wish. The fitted socket connects to the relevant circuits as you would expect.

Thanks Badger and Dougal, I like the old systems!, I was just worried I'd blow something up. It took me an hour to locate and disconnect the sounder that beeped every time the front door was opened and driving me mad. It was buried behind the fuse box on some random PCB. Andrew