Does it have to be a Koga? If so try these people: http://vintagekogamiyata.com/
The only other bike I have suitable in that size is a lovely Gazelle Champion Mondial but it's at the back of the lock up and requires a serious amount of work so will not be avalable for quite some time yet and when it is will be in the region of £350 - £400.
Many thanks for the link. I should be able to find something on there (although the first 4 or 5 I really liked had already sold)
I just Googled the Gazelle, it looks a fine bike, and somewhere down the line I might be tempted.
I have my mind set on a Koga and I am also after a Viscount Aerospace Pro- so I need to keep focused and not get distracted by other bright shiny things!