Thanks everyone for your responses - all very helpful.I guessed they were probably 10mm threads but my ex-Moto Guzzi 10mm allen socket bolts would start a couple of turns then pinch up.The thread profile is different even they are both 10mm.
I like the Burgman mirrors as they look to have much larger heads although I'm really looking at the least expensive option and think I'll have ago at making a pair of extension arms
Just had my best ride out yesterday since getting the bike in March.Geograhically speaking it probably won't mean much to our friends across the pond when I say I started out at 7.45am from Bromley, Kent and went to Leighton Buzzard (Narrow gauge Railway),across to the Glos.& Warwick Railway and home for supper at 7.30.Super A & B roads,mostly empty,dry weather,excellent machine and 58mpg over 270miles.That might not seem to be a big mileage for a day if you're USA based but in the UK its a different scenario.To give you an idea - Googlemap the route I took starting with the A41 from London - Leighton Buzzard-Winslow-Bicester-Moreton-Stow-Toddington-Cheltenham-Swindon-Bracknell-Guildford,West Wickham.
Cheers and happy riding!