Bike went into the dealer last week (SPC) to have a look at this problem. According to their diagnostics system the Hall Pickup sensor had failed which they duly replaced. The bike felt much better for about 5 miles but it appears to have slipped back into the same problem.
I am not sure if it is just me, but there is a noticable lack of power between 3500 and 5000 rpm. - especially noticable in 6th when rolling ón to overtake something on the motorway.
It is due a service in 1800 miles so i will see if i can live with it until then and get it sorted. I will try the steptoe modification as previously suggested - I think i will get it dyno'd to compare it with other stock bikes to get a reality check.
Mike