Housing Benefit stopped when switching to UC

I understood from the guides that housing benefit would continue to be paid for a couple of weeks after claiming Universal credit. I made a UC claim online but was unable to verify my identity. I then got an email from the landlord today saying I was in arrears. It's only been about ten days since I claimed UC online so it looks like housing benefit must have stopped straight away.

I thought that the claim didn't actually start until I had verified my identity, but it appears that the UC claim triggered some effect that has stopped housing benefit. I have an appointment with the job centre to verify my identity, but am unsure what I need to do before then? Shall I contact the local authority. Should they still be paying housing benefit?

I have migrated several people over to UC, including myself and have had no problem, but I've had contact from a few people who said that the housing Association landlord has contacted them to tell them they are now a month in arrears. This makes a mockery of the idea of migration, surely on the UC form, when you are giving them the name of the landlord and the rent that you are receiving in housing benefit previously, you would think there was a mechanism to let the landlord know the situation and adjust accordingly. These are the last people on Earth who can afford to get into arrears. It really needs to be made a lot clearer how to negotiate this crossover.