£600 is a lot of credit and don't forget you'll have £400 added to your account before winter.
It depends whether you need the money now for other things, or to put into a savings account. Also, is the amount you're paying now likely to be enough, still too much, or too little once the prices have increased?
Your bill should have your annual usage on it so you can work out based on current prices plus the expected 40% increase in October what your average monthly cost will be .
If you need the money for essentials, or will save it, and are paying enough for when the price increases, I'd be tempted to ask for the credit back.
If you think you'll just blow the money, aren't really paying enough for the forthcoming price increases and will struggle if they want to increase your DD by a large amount early next year because you've built up a debt, it might be better to leave it in there and just keep an eye on things.
Ideally you want to be a couple of months in debt at the end of winter, so you can repay this over summer and start to get ahead in October/November, so when winter comes around again, you use up credit and build up a debt again at the end of winter. That way you keep up with what you need to pay, but you aren't constantly in a position where it feels like the energy company has loads of your money.