I've been thinking more about what you need to do about that.
If the meter is a smart meter and sending daily readings then it will be easier.
But firstly:-
Theres been no name on the letters either sadly, theyve always just said "occupier"
That says to me that the outstanding amount is NOT what is owed by the previous occupant.
If it was then it would be their name on the letters.
In fact I would say that the previous occupant almost certainly told OVO that they were moving, gave a meter reading, and closed their account.
As you never registered your name for an account with OVO then OVO have no name on the account and so could/can only address letters to ‘The Occupier’
Are you sure that the amount owed is from a period before you became responsible?
If so then was the property empty for some time before you became responsible?
(It would also help if you told us if you are renting or if you bought the property).
If it’s a rental and there was a void between previous tenants leaving and you getting your tenancy then the Landlord is responsible for utilities during that void period, and if the supplier doesn’t know who the landlord is then that is another situation where such letters get addressed to ‘The Occupier’
The start date of your Tenancy Agreement, (or the sale completion date if you bought), is the date when you became responsible for the utility bills.
As you say, the date that you switched is known.
The meter reading when you switched is known, you couldn't have switched otherwise.
Which leaves the meter reading when you became responsible to pay.
As far as OVO are currently concerened that will be the meter reading given to them by the previous occupants when leaving.
(Assuming that they did give such a closing reading, which it seems thay did if the letters are to ‘Occupier’ and not their name).
However if there was a void period between them leaving and you getting the tenancy then the demands will also include the standing charges for that period, as well as any electricity used during that void.
If it's a smart meter communicating daily then the reading on the day that you became responsible will have been recorded.
If it isn't a smart meter communicating daily, and if you didn't record the reading at the time, then it will be a case of estimating and agreeing how much you owe OVO.
PS. Not sorting out the utilities properly when moving out and moving in is a mistake that people tend to only make once, because it does often end up messy like this.
However you are not alone and it’s surprising just how many people do make it.
Another user started a debate yesterday about the topic, and what more could be done to try and prevent it happening: