I use Dartex trousers (from when we used to farm). Waterproof, breathable and long lasting. The local saddler used to sell sets for riding in but mine are just general ones. I do ride in them - not sure you can still get them.
I like Ri-Dry, they look like breeches, are slightly "tacky" in texture, so you don't slide about all over the saddle, and you don't heat up inside them!
Joules did some nice ones last year - not sure if you can get hold of any this year as don't know if they are still making them. Got a pair for about £15 I think lovely warm and waterproof and a bit tacky too so not too slippy for riding!