Thanks guys, that‘s all good info. We generate about 2-3Gb of traffic a day here, so it‘s a challenge to find a host that can handle it without breaking the bank.
Still, there may be some merit in the "secondary server" option, I‘ll check it out.
As a side note, while UBB (the forum software here) is commercial, we get it for free because this site is on the beta team. Means a bit more work to run through their test plans (and the odd hiccup for you guys) but cuts the cost, which is good.
Thanks to everyone for your support.