Thanks. I'm just wondering what the pro's would use. Looking at this product: http://www.screwfix.com/p/water-level-kit-10m/18133 It appears that its impossible to use as is for low level work - like paving stones, this would also apply to my job as the bottom of the padstone is just 75mm above the lowest level I can work at. I guess a work around would be to attach the top of the level to a piece of wood and mark it with a scale going down, so the top of the level can sit above the work area.
Seems crude, but should get mm accurate - I guess a floor thats with a few mm in each corner is pretty good.