Well, finally managed to borrow a G- series 14" pitch prop yesterday, and went for a spin.
In summary - 4 up + 30L fuel - 4500rpm, one up + 15L = 5500rpm.
So, assuming the pitch increase of 2" takes out about 400rpm, looks like the extra inch in diameter accounted for 600 rpm (using the 1-up rpm compared to the original test).
Unfortunatley no GPS this time as it was all a bit last minute and it was left behind (d'oh!)
So, if anyone has a 14" G- series that needs a good home......... (or a 13" for those days when I have passengers!)