UPDATE
So, it's very clean, cost £1400 for a 1999 model with 22K verified miles. 3 days and 150 predominantly motorway miles in the saddle with the following observations:
Pros
Fun
Fast and manoeuvrable; filter through the narrowest gaps
Don't need a ladder to get on it
55mpg over 150 motorway miles at approx average 80mph
Research suggests cheap and cheerful to maintain
Cons:
It's not a GS
No regrets. It's not going to win the Paris Dakar or first prize in a beauty contest but now I have the best of both worlds.
(P.S. Anyone want to buy a Vespa?
)