Hello Geoff,
The condition of the final drive on a ST1100 will depend on how well serviced it has been , when I purchased mine in 2013 the previous owner told me he did all his own servicing, all he did was change the oil and filter, the bike required new tyres also ,when the mechanic I use went to change the rear tyre it took him 40 minutes to get the rear wheel off the spline because it had not been greased properly , the spline was so badly worn it would have failed within 2000 kms had I not taken it in to be serviced.
My bike is routinely serviced and maintained , it had 65,000 kms on it after that service and now has 103,000 on it with no problems.
If you can’t get to see how it was maintained prior to purchase get it in and have someone go over as soon as.