Thanks Louis - they are around. The scale card used to do them before they closed down. I have come across ones that can be printed out but they require laminating
Mate thanks for the link - that's where I ended up in the end. Bought a couple of their rulers and they will do. They are made of styrene (I think) with a printed guide stuck to the top.
I'm a little disappointed as I thought they would be metal but for the price they will do for the time being.