Brilliant, thanks so much!
Penultimate question, is this the stuff?
Fibreboard: http://www.wickes.co.uk/Wickes-Premium-General-Purpose-Underlay/p/215589
Vapour Barrier: http://www.wickes.co.uk/Wickes-Green-Polythene-Vapour-Barrier-2-5x20m/p/153230
Last question, how might I go about levelling...