Hi Shaun,
Just browsing and I saw your post mate. I thought I'd offer my similar story.

I had a problem awhile back with my GU. A knock only distiguishable just as the vehicle motion stops. I could hear it as Ive had the vehicle awhile, but it could easily be missed if not paying attention.
At any rate I replaced bushes pushed and pulled flexed and swore and basically chased my ass for awhile until after considerable time spent underneath I noticed a slight witness mark on the gearbox mounts and then noticed how sagged they really were. It seems the gearbox mounts were worn to point of making a slight knock as it came to rest.

I replaced these myself and problem solved. Good luck, I hope this helps.

Frosty