At summer I normally nil my balance if it's in credit, giving me time to build up any credit for winter. I use my annual usage to set my direct debit - so if I use £900 a year, I would set the direct debit to £75/pcm and not alter it unless I noticed my usage was rising all of a sudden when I do my monthly meter readings.But I have 600 credit. It'll only go up to about 130. And with the 400 that will Cover it.
130 is only 50 more. That would last 12 months of winter months. Not 6
I'd rather them return the cash and up the DD. Not many people carry 600 of credit.
Of course, with the prices going up in October, I would take any credit it, less a month's direct debit. So, using your £600 balance and my £75/pcm as an example, I'd ask for £525 credit back. If the supplier resists, I tend to remind the creditor I can trust them with my credit balance for 6 months, so why can't they do the same? This usually makes them see sense and I see my credit back usually 10 working days later.