If you can check out something like this;
BRITISH ARMY IMMERSION SUIT LARGE - YELLOW AND BLACK WATERPROOF SUIT - GRADE 1 | eBay
They are usually the breathable Typhoon type with reinforced seat and knees. £85 is almost a comedy price compared to new, they are tough and well designed. Bought one 2 years ago and have not regretted it, my "used" model was still in its bag with braces still in elastic bands - so brand new in effect.
For the money I'd be tempted to buy a spare. My latex socks have gone so might get boots put on, I use Typhoon rock boots at the moment, they can be a bit chilly in the cold if your boots are getting wet all day long.