Okay I advanced one step. For the computer to see the device when in fastboot, it needs a different driver. Go and download it here: https://developer.android.com/studio/run/win-usb. Unzip the driver file somewhere you can remember. Turn off phone. Start it in fastboot (hold Volume Down + Power button). Now in Windows open Start Menu and type devmgmt.msc, and locate Other devices > Android device. Right-click this and select Update driver. Point Windows to the driver folder you unzipped earlier. Confirm. The driver gets installed. Now in the Windows Device Manager (devmgmt.msc), you should see Android Device > Android Composite ADB Interface. This is what the phone looks like to your computer when the phone is in fastboot state. The ubports installer will now detect the phone when needed. Ubuntu Touch was installing while I was writing this and it is now booting in there for the first time! Language selection screen is up! Good luck

@sirosisofliver Nice!

Hi thanks for the reply. Unfortunately I am a "weekend warrior", work commitments leave me with little time to tinker with it, but when I get some at weekends , I will try again. I have now some non fully working phones to test (bad motherboards etc) against and I will try another version of TWRP (I tried the latest) to see if that works better. I am just hoping it is not coincidence and either the RAM or (not CPU) eMMC has failed (I have access to a reflow machine, but with COVID I don't know when. Thanks for all your suggestions, when I have some spare cash I will start donating to UBports as I think it is a really worthwhile cause, anything to stop hardware going into a rubbish tip.