In reply to DMR: Thanks for your views folks.
After experimenting with various brands and fabrics, I have decided that merino offers the best all round solution for me in terms of thermal properties, wicking and general comfort (not to mention odour control...).
On balance, it would appear that a zip neck offers a decent compromise between extra ventilation (zip down) and draft exclusion (zip up) as conditions dictate.
I suppose my next question is - does the versatility of a zip really justify an extra £20 on the price compared to the crew neck? Seems like a hefty sum - or am I missing something?
Thanks again,
D.