This post details how Bulb calculate the recommended DD payments. Just a comment that most of the time the email is a suggestion or recommendation to increase/decrease your DD payments and not an attempt to notify you of Bulb unilaterally increasing your DD payments.
Can’t comment on the rise it self, as I do not have the details, and i am not suggesting you post them either.
There has been several threads on this matter, you might want to go an search the forum, to review the comments, and suggestions how you can manage DD.
As long as your in credit and you have enough credit to pay for your monthly bill, there is n’t much of an issue, the only time Bulb will unilaterally change your DD if your in debt or will be in debt in the following month.
You can ask for a refund of credit if your account accrues a considerable high level of credit, but with the DD payment scheme you will at one point be 3 times your DD in credit. The credit is to cover the winter months or high usage months, as the DD is calculated on an average monthly cost, compared to paying the bill on demand.