Hi OP,
This current energy pricing crisis, and now your post, are both reminding me of the job I used to do when I worked for British Gas, which was to deal with the excessively high bills and the zero bills.
Not sure what supplier you are with, and procedures will have changed since my day, but I'd say both you and the supplier are equally at fault here. They should have a procedure to deal with zero bills, and you, having noticed your £0 charges, should have reported this straight away.
I understand how some PPs have compared it to scanning an item and it not charging a price, but what you are doing is the equivalent of going back to the shop and "buying" an endless supply of the free item before anyone notices, i.e. you know it's an error and are exploiting it for your own gain.
I doubt if meter readers are trained to inspect meters any longer, and smart meters will rarely be seen by a reader anyway, so the company should be picking up on this at the billing stage.
You, however, know the cost of gas and if you are using gas, you know you should be paying a bill so you should keep yourself in the right by reporting it. If they fail to act, that is not your problem - you have at least (and at last!) done the right thing and brought it to their attention.
Re. the backdated charges, IIRC, they cannot force you to pay anything at all, but given how long you have left this, refusal to pay a penny may be interpreted as you wilfully avoiding payment all along. The usual outcome when I did this was to negotiate an estimated usage and amount with the customer, and agree a one-off tailored payment plan if required.
And your non-indicating meter should be replaced with a new one, unless it's just not been set properly in the first place.
You wouldn't have got this far with it in my day!😄