So do i tell them and have to pay all in one massive bill or just forget it
Well it's not your responsibility to update the industry database, but....
How you behave does make a difference in fighting any massive bill which arrives in that by being "knowingly evasive" you weaken the (good) case you currently have to avoid or mitigate a massive bill. My advice is to bring it to your suppliers attention - once - but it's important you make very good notes of who you informed and exactly when. If you are ignored in time this will be to your advantage. If you are sent a "massive" gas bill, demand easy payment terms on the grounds that the database was not timeously updated. If you are refused terms, complain strictly in accordance with the supplier's complaints procedure.
Did you call to give the electric reading in response to a request or in response to a bill? Do you know how often the supplier is issueing bills or statements? Normally every 3 months, so was there an electric bill after 3 months or is this your first?
Do you realise that probably you would save money paying by fixed monthly Direct Debit on a dual fuel tariff? If you haven't done so recently it is well worth doing a comparison on a Consumer Focus accredited comparison website.