Hi old Dave
It IS a bit delicate, I agree, but I think most people understand.
When we did our very first demo at the store here 3 years ago we happily let the guy wander off up the road and then realised....duh! what if he doesn't come back? (idiots or what?) Luckily he was a nice chap and of course he did - and he bought a bike too. Now we ask customers to park in our car park and ask for their car keys, or leave us a 'hostage' (eg wife or mate, although we have in the past been left grannies, a baby, also a dog and numerous suggestions about mothers in law....you can imagine.) Nobody has taken offence to date. With personal demonstrations its a bit different though, because even if you have their car keys, they could still fall off and damage your bike and themselves! I suggest letting them sit on it, poke and prod all they like, then get on the bike yourself and ride up and down a few times in front of them so they get the idea.
Hatti, Woosh Bikes