Does anyone know of any shops where you can buy old fashioned weighing scales - the ones where you have two sides, one for whatever you are weighing and the other for lots of little weights to balance it out.

I've seen one or two at the Elsecar Heritage centre at the antiques emporium there. But I don't think they're particularly cheap.

Argos have some

http://www.argos.co.uk/static/Product/partNumber/8427324/Trail/searchtext%3EKITCHEN+SCALES.htm

i have got a green and brass vintage cast iron one on my website - see below, it doesnt have weights with it though so you do it for £15 :-)

my mums got some of these, you could perhaps try charity shops. might be lucky there. Or have a look on ebay??