Well, I spoke to customer services which was a painful experience in itself!
Turns out they will upgrade me for £24.95 per box. I asked about having an additional mini in our 2nd bedroom and was told it was another £10pm on my bill and £79.95 charge. The guy didn't sound too convincing, so can anyone confirm this is correct?
It might be free if they invite you and a charge if you request it.
Mike