Call them first!
I don't have the Darien Trouser, but have an roadcrafter suit.
Similar to yourself, not sure what size to order. Especially since it was mail order from the US, and it was quite a bit of money.
I decided to call them. The lady that took my order was very good. she asks you about my regular clothing sizes and then ask what I wanted to use it for. They then recommended a size and it fits exactly how they described.
I would recommend ordering by phone and have a chat with them before deciding on the size.
T.