Tarq57 - I've used TCP Optimiser and it has made a small but noticeable difference.
call100 - I just have, and it got rid of 40mb or so!! Strangely, afterwards, I found that my Download Helper add-on was missing from Firefox. I've reinstalled it, so next time I use CC Cleaner I will probably remove the tickboxes from Mozilla as I have it set to clear Cookies and History etc on closing anyway.
By the way, after my experiences with Net Limiter I have found a far better solution - "Bandwidth Monitor". It takes up much less memory and does exactly what I want. It is surprising how much bandwidth even ordinary web surfing uses.
Once again, thanks everyone.