For anyone interested, I was down in Poole last week and a local army surplus place - "Frontline", I think - are selling cotton (?) smocks for about 25 quid. If you fancy a basic design with a hood and large front pocket that you can either proof or alter to your own specs, this might be a fairly good low-cost option.
They came in black and olive green and another couple of colours I can't remember.
They came in black and olive green and another couple of colours I can't remember.