I just changed the engine over on my 300tdi Discovery.

The new engine had a different alternator.

I swapped the old one back over but failed to record the locations of each wire as I removed them.

The large feed wire is obvious. the second wire with the strange captivated nut is next. The third pole I think has two ring terminals on it. On the old alternator that was a single spade connector.

So, from left to right . Big brown feed. Brown yellow. Black (single) White (single)

Have I got this the right way around?

Brown with yellow trace is the alternator light.

Put that on the one that makes the alternator light on when the engine is off, then the other one should make the rev counter happy when the engine is running.

If it gets real bad real fast it wasn't me who told you this...

yep feed wire big ring terminal brown/yellow small terminal ya can't get em wrong. white is rev counter (if fitted)