Thanks. I thought as much. We've been offered a fabulous deal elsewhere but would prefer to wait till that's installed before cancelling Virgin but don't want to pay for both for a month.
Overlapping does cost, it can make sense. If it's just reactivating a not-so-old Openreach line then you might take a reasonable risk, if the line's really old, or you're having a new FTTP connection made then probably better to pay for a month or so's overlap. I had an OR FTTP install, done on the due date, when they left it was working, but there was stress fracture at the splice in the connection box at the house, and the new connection went down after 48 hours or so. ISP and OR were prompt enough to fix, but was still a couple of days - as I'd not then cancelled VM this didn't cause any problem, and I reckoned that the decision to pay for a four week overlap was good value. YMMV.