British Gas late charges using a smart meter

I had a British Gas smart meter fitted for my gas and electric supply. They sent me a final bill dated October for my electric when I changed supplier in September. The bill was for £18 approx but they debited £29 approx from my account. I then received a final gas and a final electric bill again on 20/12/17. These bills are for my usage from June to September 16/09/17 and my previous final bill was only for usage from16/09-26/09/17. I requested a copy of all my bills as I’d stupidly not downloaded any!! British Gas are telling me the reason for the late bills is because of national grid taking so long to reach an agreement of my gas usage for them and my new supplier. I want to know why if I have a smart meter fitted there could even have been a problem, why I’ve suddenly been billed for usage over the summer and why they debited a different amount to what is billed. I have escalated this to a complaint and they offered me £40 ‘good will gesture’ and I’ve paid the outstanding amount £100 as I received a bill telling me it was going to a debt collector if I didn’t pay but I’d like to know for future what my rights actually are.