10.6.8 update space issue!!

Hi,

So I am currently attempting to upgrade my current 10.6.3 to the required 10.6.8 update combo and when I come to the destination part of the installation process it tells me my HD doesn't 'meet the volume requirements' for this system update.. i have over 68GB free, surely this is enough? I can't find anything online outlining how much space is needed, can someone please help??