I found this link in which Bulb talk about being sometimes in debit and sometimes in surplus and in which they airily talk about either making a top up or requesting a refund. All very relaxed and no mention of huge hikes in monthly DD or two months’ worth of ‘winter buffer’ which, incidentally, were made no mention of before, during or after my sign up.
I am now terrified of Bulb forcing huge rises in DD on me which I will not be able to pay. I thought the idea, as well as saving money here, was that your payments are spread evenly throughout the year. It seems Bulb have been economical with the truth about how payments are made and, indeed, taken.