Another Spark Energy Complaint

Same issues as most, i went on spsrk thru default. They overcharged me with no proper bills i tried to lwave ans they wouldnt let me saying i owed them money. I was desperate to leave so i paid the money asking for proper bills which i never recieved. I now know i was blaintly overcharged but is it too late to fo anything about it amd where do i go

On Feb 5th 2019 I have paid £19-71 to OPOS rather than have debt collectors chasing me. I received a text (!!) from 'OPOS' advising me that I owed money. It appeared that there was an outstanding amount on my account from before Spark energy went into administration. I had no idea about this and had had no communication from either Spark or OPOS that there was an amount outstanding. I have therefore no evidence that this issue is 'real' other than the say-so of people on the phone. did verify the issue with Spark before making the payment but again have no real evidence.I have been with Spark energy since September 2018, this is because (and only because) my wife and I are currently renting and 'Spark' is the landlord and managing agents preferred supplier. Otherwise I would have nothing to do with them, and will be giving feedback about this issue. I am totally unconvinced that this is a real debt (I think it's a clerical error, my account is otherwise in order), and have asked for a follow up. It could also be a scam.....the OPOS person I spoke to let it slip that KPMG are the administrators (which I have subsequently verified by checking online) so clearly they are trying to extract as much money out of the process as possible. I haven't seen anything on this in the media......anyone else had a similar experience?
Yesterday (Feb 23) I actually received a reply from Spark (maybe?), which didn't answer my question (how was this amount calculated) at all. Instead, they just sent me a standard reply asking for more meter readings.....(see below), yet my meters were only read two weeks ago.
Their reply (extract):...obviously they are just either stalling or ducking the issue
"All pre 28.11.2018 accounts are being dealt with by KPMG, the administrators of Spark Energy Supply Ltd. Your account number 9xxxxxxxx8 falls in to this category and the administrators will contact you in due course with regard to it if necessary.
We can advise that to date due to the lack of any actual meter readings your accounts have been billed on the basis of estimated consumption. With the two traditionally highest months of consumption having now been completed this has been reflected in the estimated billing of your account 9xxxxxx5.
This estimation is calculated on the basis of Estimated Annual Consumption (EAC). The EAC is an independently generated seasonally adjusted consumption figure related to the size and geographical location of the property.
Spark has a legal responsibility to generate regular bills for each account and it is for this reason that, in the absence of actual meter readings, the EAC is used. It may be the case that this estimation of consumption is not reflective of your actual consumption and it will be very helpful in assisting us to progress the matter if you can advise us of two sets of meter readings taken at least seven days apart from each of your meters."

The thing about the estimated bill is just irrelevant. They've read the meter. Overall, having read other items on here, I am very worried about the whole process. It seems like it's quite common for Spark (or whoever) to inflate the bills or demand administration fees. Can't wait to get out of this rental!