we have got through many, many tarpaulins over the years, but have found B and Q do the best ones by far, very strong, waterproof, frost proof and thick and large enough to double them over if you need to, great value for money too
http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=9273088&fh_location=//catalog01/en_GB/categories%3C{9372016}/categories%3C{9372048}/categories%3C{9372262}/specificationsProductType=tarpaulins
3 different sizes priced from £5 to £11.98