When I replaced the tyres a year ago the primary reason was their age (13 yrs+), although I had noticed that the outside edge of the front nearside tyre had quite a lot more wear than the rest of the tyres. This time my mechanic pointed out a similar, almost imperceptible, wear pattern on the new tyres, but this time on the outside edge of the front offside tyre.
Neither of us could come up with a decent explanation. It can't
be tracking as you would get the same wear pattern on both front
tyres, and it can't be the inflation as you would then get the
same wear on both sides of the tyres. I had wondered about
camber, but that doesn't explain why it happened on the nearside
tyre for the last set and the offside tyre for this set. I guess
it's just a warped chassis (Robin Hood built the chassis without
a jig, so it's not surprising)
Solution is pretty simple. Each year, just after the MOT I will
swap tyres. So today I swapped the two front wheels. Next year
I will shift the rear wheels to the front and year after that swap
the front wheels again. Looking at the wear I think I can do at
least 4 times before the wear becomes noticeable. So they should
be good for 16 years.