Thanks for your help DX100.
No I don't have a smart meter but I did check with www.findmysupplier.energy which confirms that Avro is my current gas supplier. However, Tonik claim that the gas switch never happened and so is continuing to bill me for the same gas as AVRO!
Assuming I can trust www.findmysupplier.energy, then if I reclaim the last 2 DDs I will still leave Tonik with enough credit to pay for the gas they actually supplied (as opposed to what they have invoiced).
My only anxiety is that, strictly speaking, those 2 DDs were taken while I was still a customer and so were not really taken in error because it was legitimate for them to hold a credit balance at that time.
On the other hand, if the bank allow the reclaim then presumably Tonik cannot come after me for money I do not owe.