@SotonSteve hopefully nice and smooth
Indeed, hopefully. There have been some customers with protracted problems where old or faulty cables need re-pulling, in some cases for many months. There's little you can do to reduce the risk of that, what you can do is make sure that you don't cancel any existing service on the assumption that everything will be up and running on 18 March. It usually makes sense to allow for a two week to one month overlap when swapping between Openreach and VM connections (whichever way you move), because the additional cost is usually a lot lower value than the inconvenience of having no internet connection.