This seems to be an item that's really hard to find for some reason but finally I've found one!
www.peacocks.co.uk/lds-jersey-robe-2a7751.html
It's like a t-shirt material but a bit thicker, so nice and light, folds up small and fits around my bump fine (am 30+6....hopefully it still fits at the end) only thing is its a bit short, I'm 5'7" and it's a couple of inches above the knee when done up. £10 + 99p delivery.
Hope it helps someone!