I should have said that the previous card was 64gb and I did a full new reinstall - Etcher to add the image to the new card, then full manual setup on the new card in the Pi
Consequently it appears to be something unique to the card, I guess.
So, flashed a new 32GB card to try that - while setting it up it won’t let me install the touch screen plugin because the file system is read-only! What is going on? So confusing!
The most common cause of such problems is a Power Sypply issue where the PSU is either supplying a marginally low voltage or the supply voltage is dropping under load. I have also found that the most frequent culprit is a poor quality MicroUSB cable rather than the PSU itself.
Having said that, I have had similar symptoms with 128GB MicroSD Cards that I bought on eBay. These turned out to be fakes when I checked with SanDisk International Support who were very helpful and gave me the proof I needed to get a refund from PayPal.
Thank you, @Wheel_nut - I have now acquired a new Pi4 and established that it is a fault on the original one. It seems that the micro-SD card is not making consistent contact with the board.
The new Pi4 is working well with the same 128GB card so I’m confident I’ve sorted this.