To all of you who have replied, thank you kindly.
This is a great help, even if I am yet to make my final decision. So it seems the general consensus is that either aim for a 2nd hand Brompton or go for a brand new Dahon.
My job will be outside of London, in Kent so I am not actually purchasing the bike for commuting within London. I will be getting an overland train, no tube at all, and the main leg of the journey is the 3 mile cycle ride around the area of Sevenoaks.
Due to my lack of knowledge, I feel more inclined to purchase a brand new bike as I feel a bit safer in the knowledge that I can speak to the shop about it.
In my haste I was a little over optimistic about my height... I am just under 6" which I think is only about 1.82m ...
Thanks again for all your help
Conor