Thanks to all responders, some good stuff learned.
The ISP change over has happened, smoothly, without recourse to using my 4G mobile as a connection bridge. It was still on the old ISP at 6:35am but by 8:30am it was alive on the new. Nearly. We were sold 35Mb speed optical fibre but we are currently getting just 11Mb on both downlink and uplink. When the ISP tested our old copper connection it was able to sustain 17Mb so we haven't really seen a worthwhile improvement. I'll get the account holder to talk to them.
Incidentally, when my new 4G phone arrived last week I ran the same broadband speed checker while I was in a cafe in the village (1/2 a mile from home) and it managed 52Mb, several times in a row.
What rate is WiFi likely to be able to manage? The new modem/router doesn't have a proper spec sheet supplied.
Andrew.