Try one of these Adrian:
http://cheapestrivnuts.co.uk/tools.aspx
They are not cheap but are bloody good, solid gear. I went through a few cheaper versions before i splashed out on this one. If you use 6mm rivnuts, use the flush fitting ones, and drill the holes out 6.5mm also for added security you can slightly notch the hole before fitting rivnut this will prevent rivnut turning.
Cheers...AndyH