In reply to pete mc c:
is it new? (as in new from shop not new to you but second hand) just checking.
M24 gets damp in wet ground but thats 6+ years old and heavily abused so you can try either,
a)proofing the groundsheet
b) seam sealing the groundsheet seams (if TNF still do high seams rather than taped true bucket style groundsheets)
c) both of the above!
d) fill it with water and see if it leaks? (it will if its high seam style groundsheet so no point doing this if its that type of design)