
Simon,
I am not sure what you are trying to achieve exactly, but it seems odd to me to state that you'll be trekking in waterproof overtrousers. At -10C, it won't be raining so your primary concern will be avoiding the build up of moisture and keeping off the wind chill. In my opinion, you are likely to overheat in waterproofs no matter what magic fabric they are.
In those conditions I would probably go for a light long-john (such as Helly Hansen sport weight LIFA) and some softshell trousers, throwing on some fleece trousers and my waterproofs when I stop for the evening.
The surplus ones that Dandy MM posted look very good value, but it would be good to try them on over your trekking trousers. If you really want something roasty for non-UK dry conditions, I saw a pair of Helly Hansen 600 fillpower down filled salopettes in their Bicester clearance store for about £30 a while ago. You'd have to phone them to see if they still had them though.