I looked online for tutorials and patterns, but they all either required too long a piece of fabric, or I couldn't understand the pattern itself!
Bugger it, I'll make it up as I go along!
So I did.
I free balled the whole way and cut out various lengths of fabric. 4 bits for the straps, 2 big bits for the main bag, 2 for the poppered flap and 1 for the internal pocket. I used cotton for the outer, waterproof fabric for the inside of the straps and flap (because it seemed a bit more durable) and some polar fleece for the lining and pocket.
This is the end product. I road tester it yesterday when I took the little one swimming and it was very strong, yet comfortable. Managed to hold all our swimming gear and essentials.
Things I'd do differently next time? Probably put a couple of pockets on the front and draw out a pattern instead of just free balling! Maybe attempt it in a nice (but expensive *sigh*) Cath Kidston fabric!