UKC

Would be grateful if anyone has recommendations for fully waterproof rucksacks, 30-40 litres. The Crux RK30 and RK40 both look excellent, if a little pricey, and will fit the bill if I can't find an alternative. I'm sure Exped used to do something similarly robust and no-frills, but the examples I've seen lately have used much lighter weight fabric. Doesn't necessarily need a roll top, so long as the fabric is that heavy duty shiny stuff like Ortlieb use for their panniers. 

Cheers!

Alpkit Goudon 30. Only forty quid. 
 

No direct experience of this but have various Alpkit products that work exceptionally well 

I'm a massive fan of the Arc'teryx Alpha FL, mostly because it's waterproof and completely lacking in features. Comes in 30L and 45L sizes, but be careful, I believe the quoted size is with the 'lid' bit above the roll-top fully stuffed as well as the body of the main backpack.

I have even managed to jury-rig a diagonal ski carry on the 30L - but it was a bit of a pain in fairness.

My ortlieb atrack has served me very well.

Alpkit gourdon. Served me well for years. 

I've got a mountain hardwear scrambler, found it excellent. Not sure if tis the same as the current generation though. Stays dry inside on ghyll scrambles + rain and has stood up to a fair bashing over the years. 

https://www.lomo.co.uk/acatalog/Dry-Bag-Rucksacks.html

Although, frankly, the best answer is normal rucksack and a rubble sack

Exped do a few but pricey. Trekmates so a few on the lower end of the budget but not sure how touch they would be.