If you google "Montana 6 Groundsheet" and click on the "shopping results" then order the results by price, low to high, you'll see them starting at about £35.
Tarpaulins are a popular and cheaper alternative both Aldi (occasionaly) and BnQ do very good ones i personally have never used a "footprint" i am more than happy with the cheaper alternative and the SIG of both my monty and minny have never been damaged or really dirty, if the tarps are a little big, as long as you remember to tuck the edge's under the SIG you wont go wrong. we visit one campsite that has put down fine gravel on the pitch's all we do is put 2 tarps under the sig again no damage
------------- I Think Therefore I Am I Think? eh? If all is not lost where is it?