I left Bulb at the end of November, you took a payment in December and now want another payment for January when my account is still over £500 in credit

Generally the DD will run until the switch over is complete (up to 6 weeks) then all is credited back to your bank account.
If you have excess then I would contact Bulb direct to sort it.
When I left SP they retained an amount until the switch was complete but did credit back a portion initially after I contacted them.