Ofcom General Condition 22.14, September 2014
22.14 Where a Gaining Provider elects to co-ordinate a Communications Provider Migration on behalf of a Customer who has requested a transfer of Broadband and Fixed-line Telecommunications Services to be provided by it over the same line, it shall ensure that, where applicable, an order is submitted to Openreach, where available, for the simultaneous transfer with minimal loss of service of both Communications Services.
That's what is called (for short) a SIM Transfer. It needs the gaining ISP to submit an Order to Openreach asking for a SORN (Simultaneous Order Number). They then submit that special type of order number with their order for the move of the other service.
Openreach then have a mechanism to match the two and do them on the same day.
It sounds as though Zen just submitted two independent orders.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
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Connection - Plusnet
UnLim Fibre (FTTC). Sync 58162/14182kbps @ 600m. - BQM