These are a few websites I have found to be useful / of interest.

Open Forums (can be viewed without joining)
PC800 Forum A UK-based forum for PC800 owners with technical info
ST1100 Forum A UK-based forum for ST1100 owners with technical info

Closed Forums (you need to be a member to view content)
Britchopper forum A forum for builders of custom motorbikes and hot-rods – some amusing build diaries and useful technical info

Modified ST1100s
Adventure Rider (USA) Putting a crashed ST1100 (ABS version) back on the road, semi-naked
Insane Madfighters ST1100 Well, it’s different Link to a Dutch forum thread featuring my naked ST1100 (scroll down a bit). Google Translate gives an entertaining English version