I'm thinking of making a couple of braces for the rear of my car, but I need to know if they have to be adjustable. Call me tight and or lazy, but I don't understand why, can someone explain? Or is it just one of those, cost lots of cash adjustable racing type gimics?

I was wondering the same thing just a few days ago. Considering that you lock the nuts on the adjustable ones (so they don't move), my guess is no if you build them specifically for your car. Also,strut braces also don't need to be adjustable, although some are....

The 'adjustment' applies pre-load more than actual adjustment

That's right. Pre-load.

When I fitted mine the instructions said that one side was to be adjusted in tension and the other side in compression.

which side was tension and which was compression?