I think I have done what I wanted to but given that I had changed the database using SQL updates I am a bit worried about whether I may have subtly messed something up. I would be grateful for warnings - or reassurance!
Starting with a WP blog on which all posts had been deleted but contained pages:
I used a SQL UPDATE to change post_status for all pages to 'publish'
I did another UPDATE to change all pages parent_post to 0
As the postmeta table only contained page template data I emptied it with a TRUNCATE
Everything seems to work OK. The question is, is it OK,or is anything I have done/not done likely to bite later?