For around £30 you can get a new pair of Army goretex trousers. You wont find tougher.
If you dont fancy cammo, then the new german army gortex sallopettes in plain green are available from various places including Millers out Dunecht way as well as BA country stores. Bit higher waisted to to keep you back warm.
If you want "fishey" trousers, then Prologic or Sundridge do some good trousers.