Under no circumstances allow them to set that level of payment, unless you are of course using horrendous amounts of Gas and Electric, which for the point of this post I am assuming you are not.
Had the same issue with E-on a few months back, the figures were off the scale, i have friends and Family experiencing the same.
All of us to a man, contacted them at various points stating that the DD levels are nowhere near expected levels of usage forecasts and that you refuse to accpet their recommended DD level.
Tell them what you wish to pay to make sure you try and stay within your acceptable limits(be sensible, saying im not paying is a knob way forward!)
My own personal limit was £100/month 4 of us in 4 bed house, they finally settled on £85 a month agreeable to both parties.
Im even on a fixed tariff that was done August 2021 but the standing charge went up too!!
Robbing Bar Stewards and any excess they get from you sits in their accounts, gathering interest while they make ridiculous profits are your expense.
Be polite, be firm and steadfast.
You will prevail, some of their call centre staff are very embarrassed by their employers approach and it comes across during their calls.