Thanks for your advice.
I usually run the programme from internal memory with content on an SD card.
I have now tried with everything on newly formatted internal memory and the SD card completely removed, and also with everything on a newly formatted SD card and internal memory wiped.
Always the same result.
I guess that must mean its a firmware fault and I shall have to live with it for this trip and maybe buy a new device if it really bothers me.
Thanks again