Hi Ian, I thought at first I must have been a bit ham fisted and moved the lens carrier as I adjusted the aperture but when I focused at F5.6 and checked it at f11 it was out and when it was set back to 5.6 it was right again.
I then did it at f11 > 5.6 (out) > f11 – good again
I also noticed I had to adjust be the same amount (obviously in opposite directions) between f5.6 & f11
Therefore I am pretty sure I hadn’t moved the lens

I checked my other lenses (bought from new and carefully looked after) and no focus shift

I had considered focus shift of enlarger lenses as a historic phenomena – that modern good quality lenses didn’t (shouldn’t) do