Thanks, Bob. I've since read that on an inline four, cylinders 2 & 3 run hotter than 1 & 4. A bit like your soup is hotter in the middle of the bowl.
That totally makes sense, but I have seen conflicting views about whether 2 & 3 therefore need more fuel or less. Anyway, I'm going to downsize all jets and try a home-made version of Steve's 2-minute mod. I wanted to buy the kit, but he told me he doesn't ship to the UK.
As for the #38 pilot jets, all I can see on the forum is #35. A look at the spark plugs suggests the bike is running rich.