Hi Tom and welcome,
Assuming you've requested the FTTC solution from ADSL24, they must be using the same BT Wholesale product (although possibly with different settings).
My guess would be that this is one of the first Wholesale transfers and that the BT systems, which can be quite convoluted, and have not yet been proven too many times perhaps in your area.
With a "normal" ADSL MAC transfer there is usually no more than a matter of hours of disconnection. You might try BT Wholesale High Level Complaints.
EDIT Please see the PM I've sent you.
Kind regards,
Walter