In reply to Andypeak:
Hi Andy
I've been using one of these (MT100 Synthetic) for a few years now and it's great! Cheap (think I paid <£30 during a sale), light and much warmer than you'd expect from trying it on in the shop! Seems to punch well above its weight!
I use it for everything, from (summer) mountain marathons to pulling on at the end of a run (as I did today after finishing an orienteering event on The Chevin!) I bought my daughter one for her first MM last year (Saunders) and she wears it everywhere. Packs down small as well. Heartily recommended!