After nearly half a year of non-activity, I took the Sbike out of storage to send the wheel in for a freehub overhaul. My new LBS apparently has a trick to regrease freehubs (he says it's to do with hot grease, syringes and needles, but I suspect black magic).
Once the wheel was out the bearings felt so rough that I could barely turn the axle, yet in the bike they felt perfectly fine.
Upon closer inspection, I noticed that the cone on the drive side of the axle had somehow worked itself loose from the tightening nut and over-tensioned the bearings. How on earth did I miss that? I must have been driving over 1000 miles like that !
I took the wheel to the LBS on thursday and asked him to check on the condition of the bearings and regrease the freehub.
Apparently there was no pitting in the scales and the balls themselves were still okay too. So all it needed was some new grease. If it were a modern wheel I would be on the hunt for a replacement hub right now.
£10 poorer but another lesson learnt. Just because it rolls okay on the road doesn't mean that you shouldn't take out the wheel every now and then to check the bearings.
As for the freehub, it's back to that distinctive Shimano "tick" sound. So whatever my LBS did, it worked.