Hi Ian Great to have another Brit here!
I read ebay's terms very closely and they specifically mention US based hosting services. The issue is not necessarily having to pay US tax, it's more that you have to sign a US tax form saying that you're not a US resident and that you will pay tax in the UK etc etc because the UK and US have a double taxation agreement. Most affiliate programs don't bother until you earn more than $600 in a year, at which point they do ask you to sign it. I've come across this a few times and I'd rather avoid the hassle altogether if I can.
Australian tax law doesn't come into it because ebay is a US company and they will be (hopefully) paying me in the UK so they're not interested in where my servers are as long as they're not in the US.
As it happens I am an accountant, although international tax isn't my speciality
My Australian hosting provider did actually get back to me today with many apologies so I may give them another chance.
Isobel