In reply to Stuart the postie:
I think that was the purpose of the demonstration: to make something distinctly un-waterproof into something quite water resistant.
Funnily enough, I think the reason they actually had one of these (and it's well spotted, it is indeed a Royal Mail Jacket) was because a member of staff had a couple through previous employment with RM, hence they patched them together to make it into a reversible jacket to demo with on occasions such as this.