Everyone thanks for your replies, I have a friend who has a set of Marathon Supreme 26/1.6 so at some point we will put these on my Sherpa and see if that makes any difference, I will keep you all informed.
Relayer talked about drag of a Rohloff in his post, I am not convinced this would make much of a difference as it is a friction drag, so I did a little test, I took of my chain and span the wheel by hand spins well, free and fast, I then did the same but this time I left the wheel spinning for a few seconds and then stopped the sprocket rotating. And the wheel visibly slowed down, this point towards friction drag which I would expect to clear over time.
But again I am not concerned about my Rohloff love it to bits.
All the best and Happy Christmas.
Peter