Shortly after my last discussion with SP mentioned above where I was told there was nothing SP could do, I got a phone call saying I could be manually moved to PowerUP and I would receive a phone call that afternoon to discuss.
I asked why this hadn't been offered when I first had a problem and was simply told I should have been offerred this option.
While I didn't get the phone call but this morning I found I'd been moved to PowerUp. .
Went into PowerUP and started to purchase a Package. Despite me wanting to pay Direct Debit I was requested to enter Credit Card details before I could proceed, so I did. I was told I was successful but when the screen updated, no  card showed and I was still being requested to enter a Credit Card. I entered my details again and was told my card was invalid.
So I entered a 2nd Credit Card. I was told I was successful but again when the screen updated, no  card showed and I was still being requested to enter a Credit Card. I entered my 2nd  card details again and was again told my card was invalid.
At this point I gave up, logged out then logged back in. Lo and behold the 2 credit cards now showed.
I then proceeded to purchase a Package but the App advised PowerUp was having trouble retrieving my packages. I should contact PowerUp.
I've done that and am awaiting a call back in 6 days time. Despite PowerUp advising I owe 2 days energy, I'm not worried. I know I am over a month's energy in Credit with SP.
Looking at other Posts on this site, it appears many people have decided to move to PowerUp when their old Tariffs expired 31 Jan, so the PowerUP team are being overwhelmed with problems. They all have had similar problems and all been given similar "We don't know how to fix it" answers. However within the last few days, their problems have been resolved and they can now access PowerUp. Not sure if they also are getting further problems in the actual purchase.

Hi DHogg, fascinating sequence of events, the App can barely be called Beta just yet. One thing you could try; put in meter readings and see if that jogs the system into accepting a payment.

Went into PowerUp App this morning and I was able to purchase a package. All seems resolved for now.
My brief analysis of my changeover (and feed back from other Posts) to PowerUp (from the perspective of a Business Analyst/Controller of a similar billing system for a company dealing with 4 times the population of Scotland.
Although launched in Sep 2016 PowerUp App is still BETA and upgrades are not being fully consumer tested.
HelpDesk staff are not being given sufficient information on PowerUp and/or its problems resulting in

• wrong information being given to customers
• useless reinstalls/retries by customers on the above advice
• increasing customer frustration that nothing works, no one knows what's happening and many promises of call backs and emails from SP are not being actioned

The above are not the fault of HelpDsek staff.

Short term Solution:  Give the HelpDesk staff give the information they need to help resolve the problem at first call. Then customers won't get frustrated and Tech staff won't be overwhelmed with demands for call backs and emails they cannot meet.

Long term solution: Ensure App is fully consumer tested and that new versions are robust and not device or operating system dependent as much as possible. The last part may dificult difficult if manufacturers stray from strict software standards and is beyond the control of developers. Our solution was to have a select group of volunteer tech savvy consumers with varied operating environments who were given dummy accounts or access to a test environment when we needed to test new versions. In House test scenarios need to be rigorous and exhaustive

Hi DHogg, Glad your PowerUp is working now, lets hope others will read this thread and that it helps them. Your comment about Smart meters was interesting, I have been asked if I have recently had a smart meter (I haven't) because this apparently needs manual intervention before it starts to record on PowerUp successfully.

Nelson, keep us up to date with your progress.

What a load of rubbish and appaling customer service from power up noome to talk to 5 days for call back. Rung up to make a complaint told I would have to wait 5 days for a call back to raise a complaint app not been working for over a week had messages to top up yet I’m over £500 in credit with them now show my accounts have been closed sp really need to sort there issues out and have a customer service department for a product the6 are quick to sell 

Hi jaki9981, this has been reported by another user in the Community, Eilidh who is a moderator for the Community may be able to help you. It is a computer problem and I am sure you will not lose your money.