I would suggest the installer, misread the numbers on the meter, when (s)he initially installed it. They usually ask you to witness the numbers, and sign a card to confirm the numbers, did they?
Neither the swap of faulty (stuck display) meter by EoN for a dumb one, nor the more recent swap to a smart meter required me to witness or sign anything.
They did affix cards on to the meter back panel in the external meter box with old and new readings in both cases.
Naturally I'd taken pics of before and after in both instances. I assume the OP did, too?
Likely by moving into a property the OP has entered into a 'deemed contract' with the incumbent supplier EoN? OP needs to seek Legal Advice (via house insurance) and go the energy Ombudsman route as well if the complaints process has been exhausted / reached deadlock?
EoN's meter changer probably has electronic records and pictures taken... ask for copies of those?
The basic 'turn everything off' and monitor the meter reading(s) should be done (if not done already)... some installs now have an isolator between meter and consumer unit to do that easier... Nothing on means meter should not increment and the red light will, after a period, go steady red. {It's the fault mine had}.
Similarly switch off unplug all unnecessary circuits and appliances in case something is eating electric (electric UFH, immersion heaters, pond pumps, hot tubs, pools).
Another step might be to get EoN to install a check meter?