Thanks all - a miserable experience but now I almost preach to anyone who will listen on the virtues of being ultra careful.
The anti theft chain in theory could be detached by the thief but it would take time and precision so unlikely to happen.
Finally when I am out say for a coffee I take the backpack off place it on the floor beside me and put my leg through the strap so that it can't be surruptiously pulled away. If I have it on the seat next to me I cover it with my coat.
Inspector Clouseau would be proud of me.
Six foot two, eyes of blue. kg v lb. cm v ft + ins