Thanks for your thoughts. This is the compound I was looking at:
http://www.wickes.co.uk/invt/154134?utm_campaign=bazaarvoice&utm_medium=SearchVoice&utm_source=RatingsAndReviews&utm_content=Default
I was thinking I would only need 2 bags to cover an area of 1700mm x 1000mm x 15mm depth.