After a number of punctures in the rear wheel over the last week, I have attempted again to diagnose the cause. Worthwhile, as on day after work, I had to walk for 2.5 hours until my wife could pick me up (I wouldn't pay for a taxi as that would have been most of that day's pay wasted - it would have been at least a 6 hour walk in total, if I'd had to walk all the way home).
For the umpteenth time this week, I have gone over both the tyre and wheel
millimetre by millimetre and found
absolutely nothing. I have gone through my entire store of tubes including after having patched a number of them. I have also checked the rim tape and spokes, nothing poking through, not even a burr that could have ripped a tube.
Having put the job off until today, as I have been extremely tired all week, I had what may be an epiphany. What if I was using the wrong size tube?
Bear with me.
I used to run 32mm tyres on my bike so had 28-38mm tubes. I then switched to 28mm tyres and kept the same tubes. What if the 28-38mm tubes were too big and being pinched in some way shape or form when going over a bump?
I was that tired the other night, I bought the wrong size tube in
Halfords (I had ridden 23 km to work, worked all day, ridden 2km then walked for 2.5hours pushing my bike, so I'm allowed

plus running round like a blue-ar$ed fly for the previous 3 days

). I bought 23-32mm tubes.
This may be it says I. There'll not be any extraneous material (although inflated) to be pinched. So I've installed a 23-32mm tube into the wheel. I've taken the bike for a test ride (only 6 km or so total), purposely ridden over a few bumps, and it
seems fine.
I did have a issue where the bead didn't seat properly causing a flat spot. A dab of washing up liquid sorted that out.
Could this be it? Could I have solved it? Could I have been so tired, I couldn't see the wood for the trees? I don't know. Time will tell I suppose.