David Hearn wrote:

Our bath runs under the window and has a shower fitted. At the moment we
have nothing at the (frosted) window - and therefore showering is a tad
risky due to the bathroom being at the front of the house.

We don't want to put a shower curtain down the window side of the bath as it
would narrow the shower quite considerably.

Ideally we'd like to still be able to have access to the window ledge
(useful for putting things on) - however, it would also be nice to stop
water sitting on it. My thoughts were to put a plain roller blind against
the window - leaving the ledge free - however, I've no idea about how roller
blinds like water! With a curtain, it drains nicely - but with a blind, you
couldn't (I expect) roll it up until its dry - else it'll go mouldy.

Any other great solutions for this?

What about a short, flat-to-the-wall waterproof window curtain, rather than
a full size shower curtain? With weights at the bottom, to keep it flat.
It could be cut down from an existing shower curtain, or you could make a
matching set from waterproof yardage.