Is he looking at a new fix to replace one that is ending? Chances are that it is much more expensive than the price cap, so don't sign up until you've read the advice by Moneysaving Expert.
www.moneysavingexpert.com/utilities/-are-there-any-cheap--fixed-energy-deals-currently-worth-it--/
But even without signing up to an expensive fix, it might not be far off sadly. If you're coming off a fix set 2 years ago, the price cap was around £1050 or so and there were fixes available quite a bit below this, around £900, based on typical use, which it sounds like you're quite a bit above.
Now there are no cheap fixes and the price cap come October is going to be around £3000 so 3 1/3 times higher. That ratio multiplied by £1800 is £6k, so the extra £1k could be explained away by a debt on your account or your company setting your payment a bit higher to prepare for higher winter bills.
You need to fully understand your annual use and how much it costs, not just look at DD figures to make sure you pay the right amount.
With that consumption, you could also benefit from seeing if you can reduce your usage, by changing behaviour and/or investing in energy efficiency measures.