Oops posted on the wrong thread previously
Datapoint. Yesterday a Sunday I did that trip at around 1530 and as I passed the dock gates at Rosyth my time from Ocean Terminal was 50 minutes, The route is such a lottery. So allow plenty time! That was an average of around 18mph just heavy traffic on one part of the route for about 4 miles and a tail back on the bridge for no known reason - may have been a problem further up the motorway. So allow plenty time would be my advice.