B & Q do a few of these of different sizes, might be worth having a look on there website.

Thanks for the replies guys, I'm after one I can wheel into customer sites, I've got a top box in the van, but I'm after a mobile toolbox, to cart round tools up and down aisle etc etc

Seeing as your in the UK, have a look as Halford's as there Professional range is pretty good, they also come with the ball bearing runners.
The 7 draw top box & 5 draw roll cab are on package offer for £200 at moment.
There's also the Industrial range for something a bit stronger, they look pretty sturdy to but there more pricey.

Clarke boxes from Machine Mart are also good if you start from the heavy duty plus range & work your way upwards, slightly more expensive then Halfords.

get one with ball bearing sliders, but not the most high dollar, as none last in the back of a service van more than about 7 years/ 700,000 miles