My observations purely as another Bulb customer.
My gas increase as from 11 November was also more than the 18%.
There have been various discussions on this forum and also on TrustPilot and this is a reason given by James from Bulb:
"“In previous price increases we kept the price of our gas tariff lower in some regions to limit the impact for members hoping that costs would decrease. But unfortunately wholesale costs have continued increasing so we aren’t able to hold them back any more. This is why your price increase may be higher than the average.”.
I am unable to offer more regarding this comment.
However when the price increase was announced in September Bulb were quite honest and upfront when they gave existing customers 60 days advance notice of the price increase and clearly explained in their notification if you were unhappy you could switch to another supplier without penalty.
As far as the monthly direct debit is concerned you have a certain amount of flexabilty in setting the monthly amount (this wasn’t so with my two previous suppliers who were two of the big six) , you can do this via your account dashboard.
Bulb uses an estimate of your future 12 months energy usage together with the new unit rates to arrive at an estimated annual cost which if divided by 12 gives you the proposed monthly direct debit. Bulb obviously don’t have a crystal ball to know your actual usage for the next 12 months. I assume Bulb must have updated your estimated usage to account for them giving your varying suggestions on your direct debit payments.
You can if you wish ignore the suggested monthly amounts suggested by Bulb and do your own calculations if you know your actual energy usage (in kWh) for the previous 12 months. You enter this information into the Bulb quote page which will then give you the monthly direct debit to cover the next 12 months.
I use a similar approach in setting my own monthly direct debit but use a spreadsheet to work out the details. Any calculations I use take into account any credit in my account.