Early May better than later apparently. Avoid the first week of June! https://www.smidgeup.com/midges/midge-season/
The guy that I'm going with is a Scot raised near Glasgow. He came round today to revise my route. I'm working on the revisions. He's just bought a '51 BSA A10 from Denmark. Got me back to thinking about a Desmo 350 or 450 but..... Ducati old bike prices seem mad to me!
New route: On the way up we divert across the North Yorkshire moors and around Edinburgh. On the way back we stay west and then down to around Glasgow and hit the motorway home. This way is actually less miles - still a 1500 mile round trip!