DylanReeve: Design problems are no excuse for misleading information unfortunately. It seems quite likely that many customers have made the same assumption that Richard (and others here) have - and many of those people may not even realise their assumption is incorrect for many months until they have the need to top up their data.
The more I think about it, the more I think that Snap has an obligation to provide the data add-ons at the price it had to be assumed they would cost - in the absence of any other information the only conclusion it was possible to draw was that add-ons were the quoted costs for the initial plans. Basically that is has to be assumed that every customer has signed up with that expectation.
There website looks quite amateurish, and two of the top button don't work. I too read their website as meaning what others here have said, and was considering these guys as my next ISP for home use as it looked like a good deal. Their website doesn't explain it at all, even in the support pages.