How much is your monthly payment @Littlelottielulu? £384 credit is a lot if you're paying £50 pm but not so much if it's £300.
I'm new to Octopus, but they seem to be asking for monthly meter readings so you should see your credit dwindle over winter with each monthly statement as you use more than your monthly payment, but then this should balance out as we go into the spring and you should need to use less heating and you might even start getting ahead again.
Unless you use a lot of energy, eg more than £200 pm at your current prices, when averaged over the year, I can't see you building up a big debt by June.
You're quite lucky, it is likely that the unit rate and standing charges for your fix are below the price cap and anything available currently, so you should stay where you are and just keep an eye on your balance when you get each statement to see how it's going.
But yes, unless there's a big drop in wholesale prices, you need to prepare yourself for a big rise when your fix ends. It could be that you'll be best letting your account go onto the 'variable' rate, which at the moment should be regarded as a 6 monthly fix at the price cap, which is currently below the cost of supplying energy and cheaper than any fix available, but who knows what's going to happen over the next few months with both wholesale prices and also the price cap rules/system.