OK, two things!

'Why?' is I suppose, a valid question, as long as it's not a thinly veiled attempt to suggest the asker doesn't know what they're doing. To answer, I'm copying some program code into ram which will erase and re-program the flash 'on the fly'. (Because I want to)

Secondly, thanks for all the suggestions. I assumed (incorrectly) that the PSEN/RD logic was correctly fetching instructions from ram after the switch was made. It wasn't, but it is now, and it's working fine!

Thanks again.

David