Originally Posted by barker71:
“I see that the one plan has unlimited data if you tether from a phone,”
Yes it does.
Originally Posted by barker71:
“am I right in thinking that you cant use the sim in a dongle or mifi?”
That's right.
Originally Posted by barker71:
“I live somewhere where 3g broadband is the only option so really grateful for advice.. happy to accept pm's if you don't want to post here!”
If that's really the case (which I'm quite surprised by), then one option would be to get a phone that can act as a Wi-Fi hotspot (the iPhone 4 or newer and most Android devices can do it), allowing you to use the tethering on several devices at once.
While that would be permitted on "The One Plan" it's not really what they intended the service to be used for, and in the past they've tried out some traffic management schemes to discourage it.
A mobile network just isn't cut out to replace a fixed broadband product.