You should look at your bills and get your average usage for both gas and electric. You can then calculate this off the unit rates that they are quoting you (they need to give you unit rates, not just dd rates). You can also do it based off the 64% increase on the current variable. Once you have done this and have your average yearly cost you can then divide this by 12 and get a more accurate monthly dd for yourself.
Lots of companies are either serverely over estimating or severely under estimating dd's. This results either in lots of credit sitting with them (them getting interest off it) or people ending up in debt
Submitting meter reads in March threw loads into debt as they had been paying quoted direct debits and then being billed estimated costs
There's a fantastic group called Energy Support and Advice UK on Facebook and it's been a godsend for me!
I'm by no means an expert at all tho, you'll get lots of good help from people there