The differnce is always worked out in the customers favour and as someone else has said it is pro rata.

Also you dont have to have fyour account on 6monthly billing you can opt out of it!

If you have 3montly you get 2 estimates per year and 2 actual or rota reads.

With 6 monthly you are getting 2 actual reads.

I feels its easier to control on 3monthly billing so just ask to be put back to 3monthly!