Just wanted to update everyone that contributed and let you know it's all sorted! It was, as suspected, that the imperial calculation was being used rather than m3. Everything now on track. Thanks all.
I've done most of what you suggest. There own data is damning. The meter readings look correct. It is how they then convert to KwH that seems to be the problem.
It is crazy! Not sure of others having same problem, but I do agree there must be more having the same issue. What's even worse is I've already taken this on twice before with OVO and basically been told that it is what it is. It's only now that I've really done my homework and THINK I'm onto...
I think they realised there was an error and put a hold on the account for 30 days. Not really sure what that means other than perhaps no further upward movement on my DD from the recent increase to £590 per month!!!! They're so kind