I guess anyone looking for a used Thorn tandem will have checked out the Tandem Club UK For Sale list. There are several nice looking Thorns there at the moment including a 1/2 and 1/2 S&S coupled Adventure for a fairly tall crew. That's the only one I've seen with the couplers at mid point apart from ours which we bought within a few hours of it being listed. No regrets there.
Another place we found a good Thorn tandem was FB Market Place. There is good looking Adventure on there at the moment located in Newton Abbot.
I would take off the dreadful SE brakes and put on XT V brakes with appropriate levers, Tektro RL520 for the drop bar bike provided the captain has medium to large hands.
Martyn
Shame to see these nice bikes sitting there but our tandem fleet already well out of hand.
