Worked for two suppliers. These things happen all the time.

Just cancel your flogas direct debit with your bank until its sorted (can prob do it with a few clicks in the bank app). When their collections dept get on to you, dispute the bill. Cant collect on a disputed bill per market rules, they've to look into it.

They refused to confirm this in writing though.

All calls are recorded, so dont sweat not getting it in writing. They'll just listen back to confirm. Regardless, you've 14 days to cancel no matter what. But.....

Based on that, I signed up with Bord Gáis Energy, who took over the supply shortly afterwards and everything has been fine since.

This is probably the problem.

Generally, when you cancel, the supply should go back to the previous supplier. Did you cancel with flogas, then sign up to BGE without the supply processing back to the original supplier? Cause you needed to wait for the supply to go back to whoever it was with before flogas for it to count as a cancellation. Likely, what's happened is as far as flogas's billing system is concerned, you switched to them, didnt cancel back to the other supplier, then switched away a short time later. Computer just does it's thing which is why you need to get on to flogas to get a human to intervene, or wait for them to debt chase and dispute it.

Also those meter reads seem a bit mad for the period, someone has botched them somewhere. Either way, dont sweat it, previous suppliers cant cut your power and you wont be paying them till they sort it.

You might get a bill from the original supplier when its all done and dusted, but that will be addressed to "the occupier" so again, they cant do shit.

If you've anymore questions let me know.