Thanks for your input gents. Without doubt there is consensus, both here and with people I've been speaking to in the phone for 12mm. And I think to start and keep doubles on the radar for a later date.
Just talk to me a bit more about larger sizes though. One or two people on the phone seem too think that 14 would lead to a pen that is really quite chunky. That may be because they don't like chunky pens. But why do you favour 14 over 13?