In reply to pak.pako:
I considered the Decathlon £100 overtrousers. I was put off because the hydrostatic head appears to be the bare minimum of 5000mm, and I found a few reviews on the French Decathlon site that said they weren't massively waterproof in the rain, better for snow. I have no personal experience of them though.
I went for the Keela Alpine Advance trousers. I've not used them anger yet, but first impressions are good; high waist, full length zip with stormflap, integrated gaiter, double layer fabric so should be warm, and the Munro jacket in the same material gets good reviews for waterproofness.