Hi Nick, welcome, sorry you didn't get a reply sooner.
Whilst bikes are 'sized' on seatpost height, often the measurement of the top-tube, i.e the effective length of the bike is more important. In your particular case reach will be more important than traditional frame size (seatposts can go up and down a long way to accomodate leg length but reach adjustment is much less adjustable.
A good LBS (local bike shop) will be able to sixze the bike to fit you. A really good shop should have a measuring service or a jig for you to try. You might have to pay for the service but it may be worth it for proper bike fit (often the fee is discounted from a full-price bike, typically £30 ish).
Good info about fitting a bike can be found here:
http://www.peterwhitecycles.com/fitting.htm
You might save a few quid online, but a badly fitting bicycle is no joy. A good bike shop should set the thing up for you to try.