In reply to olly.climbs:
I have just bought some Toughprint inkjet paper A4 £17.99 off Amazon(comes in laser paper as well) I signed up to the OS free trial 1 week, this allows you full access to the maps and allows you to print them, for future use there is a £3.99 month long payment that would be fine if your taking a trip somewhere new.
I printed of a good quality Pixma Pro 100s with canon inks, I didn't try on my cheap printer with generic ink (although I may try to save on the canon inks).
It is worth noting that on A4 1:25000 is quite restrictive especially if you have printed of the map before deciding which route exactly your doing that day, I do print the 1:50000 on the reverse side so you should have enough info to get back to the car park
(OS fits to scale )
I haven't put the maps through the full wet mountain day pulling out of pockets and scrunching up etc, but I have left a copy outside in the rain then given it a not too gentle wipe down and it was fine, paper pretty resistant to tearing and folds well without the creases ruining the map.
This thread gave me a bit of a nudge because I did my ML assessment recently and found the whole map case thing a real pain in the ass, the assessor had a bunch of old maps he had cut up and laminated so they were a convenient pocket size, but this if it takes the full mountain test will be better.