Thanks for the replies. I'll most likely be using them after good reports on other channels.
I've just in the process of re-sealing some of the seams on the fly which leak in heavy rain, so will give the fly a rain proof test before making the decision to splash cash on the ground sheet.
Its an absolutely bombproof tent and a new one would set me back ~700 quid so 100 quid to get a good quality new ground sheet is a bargain, (if the fly is still good).