Yeah, their NAS backs up to mine via the internet and vice versa so it's probably the biggest consumer of data...

Thanks

then you're right to worry my local NAS can clock up ...up to 500MB or 1GB on a 'bad' day !

Do you have any traffic monitoring? Can you see how much data their NAS sends to yours (and how much your NAS sends to theirs?) - it could be a good start!

Once you get the usage figures, what are you looking to do? Save some money by going somewhere with a limit, or move to a more specialist ISP (e.g. Zen or AAISP) once you're sure their usage wouldn't break the bank?