It was explained to me that the £5 rate is per month for the entire duration of your contract. The £7.50 rate gives you the additional flexibility to add / remove the bundle on a month by month basis with no commitment.
I cancelled my Voda contract as it was second one and not a heavy spend. Now I'm looking for a single contract for myself, I thought Voda might have offered me something, I'm just getting the same deals as new customers. So I'm looking at the O2 simplicty £20 deal - everything I need including internet.