Go to Wilko or Robert Dyas and buy a small piece of anti-slip mat to put under your ground mat. You can also find it on eBay.
It's like a sort of rubberised mesh, for want of a better description, it weighs little, and it rolls up small to fit in you rucksack.
To save a few grammes I later I sprayed the underside of my Multimat with anti-slip treatment. While it is very effective I do not wholly recommend this, for it has remained permanently slightly tacky.