All fixed! I had a small chat with the transmission guy before handing over the keys, armed with a printout of the same FAST schematic as above. The problem proved to be exactly the same as identified by SteveL and experienced by cowie165: broken spring (part 32185) which was locking me out of reverse.
I also had the helpful guys at Automac Transmissions replace the oil with a gallon of Redline Shockproof Lightweight Gearbox Oil.
Result? One full-functioning, smooth-shifting gearbox.
Cost was a very reasonable $85 for the labour/ parts (not including Redline Oil, which I bought elsewhere).
The gent. who replaced the oil said it was full/ dribbling at slightly under the 3.784L - which suggests to me that the R32 GTS4's gearbox has slightly lower capacity than the equivalent R32 GTR....more like the same capacity of the GTS-t.
Anyway, if anyone else has the same problem, I would suggest that you arm yourself with the same diagram to aid the mechanic, and they will hopefully be able to replace the spring mechanism in a similar fashion (removing the crossmember out of its place and sliding the gearbox back slightly - just as they did with cowie165's).
Very chuffed that it's fixed - and many many thanks to SteveL and Mark!
Cheers :lol: