Andy S
That may indeed be the case but cannot be assumed. Since you mention it, though, the CAA stats are as follows for cargo aircraft as opposed to pax aircraft for 2007.
LHR 74,012 tonnes
LGW 2454 tonnes
MAN 79,141 tonnes
As can be seen, a great proportion of freight is carried at those airports by pax aircraft. Interestingly, the LGW figure shows a 93% decrease on 2006, which is odd, I think.