Evening Mr Carrot
I have both tailpack (Buffalo), magnetic tankbag (Oxford).
Was happy with the tailpack, but noticed the paint was starting to come off from the grabrails

, so I picked up a tankbag from Infinity the other day (£35) Works great, stays firmly attached well into 3 figures (on the autobahn officer, honest

), and is quick to fit and remove.
Also fairly essential if you want to take a map with you.
Croydon is not a million miles from me - pop over and you can try them out