I should have made clear that 'nearly new' wasn't the sole in the picture, I measured it after a couple of runs, maybe 20 miles.
I have a speculative theory about this, just perfect for an Internet forum -- they had an ongoing issue with injection moulding of that sole. All of my previous ones had to be binned because the lugs would start to split into layers, as if the sole was made of multiple layers that were not bonded well. Perhaps the reduction in thickness is to mitigate this. In any case, it seems to me they changed the way the sole is made, note that the injection artefacts (the little studs on some of the lugs) are no longer present on the R/B sole.
Anyway, thanks everyone for the suggestions; I'll see if I can find a shop near by that stocks Walshes to try for fit. I really hate changing shoe brands, all three of my current running shoes are made by Inov-8 (they fit me well), but the other two have now been discontinued, so it looks like change all around. Exciting times.