As an update I was able to pick up a replacement chain from the local mower shop and installed it relatively easily with the master link meaning I didn't have to remove the auto drive system.
The result was promising with the mower starting and driving both in forward and reverse. There does however appear to be some slack in the chain and the occasional but somewhat frequent clunking sound coming from under the seat when driving. As it only really occur when moving it difficult to ascertain exactly what is causing it.
I'll try to post a video of the slack in the chain and some sound (which I think is normal) coming from the drive when I had it on the jack.
