Thanks Dodgefate for the quick reply and apologies for not responding sooner. The pin you described at the rear of the tractor of course disengages the transmission, enabling the tractor to be pushed around manually. This pin was in correct position. You also mentioned the drive pulley might not be engaged but this was also where it should be. However that reminded me of something else I'd seen in another forum about how the drive belt works so I removed the battery and battery carrier and found underneath that the belt had jumped off the tensioning pulley instead.
Repositioning the drive belt was a simple task. I also gave myself a severe beating for not paying attention to the operating manual and thus avoiding the problem in the first place. It instructs that, when changing from forward to reverse, or vice versa, first disengage the blades (the old "double-declutch" trick). Anyway, all the grass and weeds around this rural property are history now and I can look forward to the approaching bushfire season with a bit more confidence. Cheers!