Having tried out various electric hub motors and finally gone for the Sunstar http://www.sunstaribike.com/ kit meant that I was now restricted to a 48 tooth front chainring. The 9 speed 32-9 rear cassette went back on, replacing the 72 chainring and 32-14 rear cassette.
Having just about managed the steepest hills the Chilterns can through at me, top speed on the flat and downhill was still an issue. Gearing was always going to be an issue on a 20in wheel.
A great website is: http://sheldonbrown.com/gears/ as you can insert a multitude of combinations of chain rings and cassettes to compare them for you target gearing.
This allowed me to come up with a range of options to price. The Rohloff was just too expensive, eyewateringly expensive. I ended up selecting the Sturmey Archer dual drive which would allow me to install a 11-32 9 speed cassette.
Will update to record if my Strava readings improve as a result.....