Quite a long post with a lot going on, so a long answer but I’ll split it into two stages:

Firstly your current issue:-
You are having problems topping up instore.

However you do also say that you have been able to top up using the app.
Your current issue is that you couldn’t do that because of a Planned outage for maintenance of the app from 1am to 5am this morning.
(TBH I am not sure why you say that you were trying to top up instore yesterday evening when the app was still live so you could have used it?)
PS. You did see the big green banner about Planned Outages at the top of every page on this forum?

Hopefully you can now top up again using the app, presuming that you have the money in the bank to do so.

If you only have cash so still need to topup instore then try the following:

When you have a new Key/card you usually have to put it into the meter for a couple of minutes so that the card can be registered to that meter before you will then be able to topup instore - Have you ever done that? (or have you only used the app up to now so have never put your card in the meter).

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Now about when your gas supply was capped off:-

You say that:

... been off supply for four years .., which the engineer told me had been capped off since I moved in.

So you had moved into a property where the gas supply to the meter had been Capped Off.

Either capped off from from the network or capped off after the meter.
Either is possible and you dont say which it was.

If the supply had been capped before the meter, on the netwrok connection:

Being capped off before the meter means that there is no supply whatsoever to the property, so there is nothing to be metered.
That being so then it doesn't matter if there was a working meter there or not.
Whether the meter battery had failed or not is irrelevant when there was nothing to meter anyway.

It is the property owners responsiblity if they want their property to be connected/reconnected to the gas network, not everyone does.
They have to request that the Network Operator reconnects, a supplier cannot reconnect a capped supply (but can ask for a Network reconnection on the customers behalf).
https://www.gasconnections.co.uk/gas-reconnection-after-being-capped-off/

You are not due any compensation for that period, it was your responsibility to have the supply reconnected after you moved in if that was what you wanted.
However as the connection to the Network had been capped off then there are no standing charges due for that 4 year period either.

See: https://forum.ovoenergy.com/my-account-140/my-gas-supply-is-capped-who-s-responsible-for-the-meter-7353

If the supply had been capped off after the meter:

In this case there would still be a network supply and the meter itself had been capped for some other reason.
It may have been a safety issue, possibly simply because the property was stood empty for a while.
(Mine was capped after the meter like that when I moved into this flat).

In that case the supplier can uncap it again, but will need to do an inspection first to make sure it is safe to do that uncapping.
And yes they do need access to the property to do that inspection.
An appliance may have been removed by the previous occupants and the pipe left open, or pipes may have been damaged during renovations. (Pipes in empty properties do also get stolen and weighed in for scrap, the metal thieves are bothered about leaving things safe).

Again it is the new owner/occupant’s responsibility to arrage to have that done, not everybody wants gas.

One important thing to note is that in the case of a cap after the meter then the standing carges are still active because there is still a network connection.
And someone is liable to pay those standing charges, either the old or the new occupier (or possibly the landlord if it’s a rental).

After some back and forth, which mostly appears to have happened because you didn't understand just what 'capped off' meant, you seem to have had a new meter fitted - but with the supply still capped off, which was later uncapped following a safety inspection.

Realistically though none of that is anything to do with your current issue, which is just that you are currently unable to top up instore.

You are a condescending individual, aren't you. Most of what you have supposed in your own head is wrong. Let the professionals have a look, mate. I don't have a card or key. It's none of your business why I was trying to top up instore and Wetherby I have cash in the bank. You even work for these guys or are you a wanna be employee? Sunday trolling. 

@stj2006 you just lost your only chance at getting advice on this issue via this Forum. @Nukecad is quite literally the single most experienced user on this Forum when it comes to basically any and all debt related stuff, yet you attacked him when he tried to offer advice that could have easily untangled this mess. We don’t appreciate that, so I’m afraid you won’t receive any further advice here - you’ll have to look elsewhere if you still need help.

As Forum Volunteers, myself, Nukecad and the others on the team do this for free in our own time because we want to help out. We do it because we can and because we care about giving folks the best possible advice we can offer them, completely independent of OVO and any other supplier. We are customers ourselves and we don’t work for OVO at all, but OVO gives us a trusted status on the Forum to enable us to do what we do best. However, we also have every right to deny offering our services at our sole discretion to anyone who doesn’t Play Nice.

You broke that golden rule by attacking my friends, so I’m afraid it’s Game Over for you and you’ve permanently lost the ability to get our advice. I’m sorry about this, but that’s how it has to be.

If you still need help, I’m afraid you’re on your own and you’ll need to look elsewhere.

~~Question Closed~~

Hey @stj2006,

It looks like one of our volunteers @Nukecad drafted a well written response to some of your points above. We manage a community of volunteers who give up their own time to try and help others and it’s disappointing to read your response to that offer of support. You can find more information on our house rules below: 

https://forum.ovoenergy.com/ovo-newbies-153/ovo-forum-community-guidelines-and-house-rules-7846

Our Customer Terms of use can be found here:

https://www.ovoenergy.com/terms/customer-forum-terms-of-use

If you need help with your account then you can find how to contact our Support Team.