Thanks for all your replies. About 15,000 miles ago, there was a very slight, just noticeable to my eyes, movement when the clutch was operated. I temporarily fastened a steel bar in front of the rad, which left a gap of about 1/4" between the bar and the fan bolts. I pushed/levered the pulley/crank rearwards, took a feeler gauge measurement of the gap between a fan bolt head and the bar, then pressed in clutch and measured again. It was showing around 10thou (3 thou more than WSM specified tolerance?) less, so took this as a rough idea of the crank float/movement. Repeated this process a few more times, and it was the same measurement.
Just did a visual test recently and the movement was again, barely noticeable, as it was previously. I am therefore assuming it is not getting drastically worse and will take the risk of it causing damage.
Dave