Many thanks for the quick and comprehensive reply. It's much easier to do things when supplied with the correct information and the reasons why.
Once again many thanks.
We've just had our loft insulated with really thick insulating material (fibre glass kind) and the chap who measured up said that, as it will be well above the joists, I will have to raise the height of the joists with some 2x2 if I want to board it out, otherwise the insulating material will...