If I buy additional bandwidth, and I correct in thinking that it will only deduct the amount of bandwidth that I've gone over my monthly limit? eg. my monthly limit is 20GB, so if I buy an additional 10GB, and one month I use 22GB, then I'll have 8GB left in the bank. Does the bandwidth bank last indefinitely? (ie. not like some pay-as-you-go phones where minutes have to be used within 6 months).
Also, there is a £10 credit note on my account, can I use that to top up my bandwidth bank? The credit seems to just be sitting there (for about 2 years) and yet I keep being charged in full each month! Seems a bit odd!