The across answers seem to be arranged as normal, albeit that 12 of them are each supplemented by an extra letter in the middle down column, being the three unclued words (eg POSSES ----> POSSESS) The down answers seem to be given in the usual order but seem to be entered upside down.
Many thanks Cruciverbali - Unfortunately I didn't see your post until today, Saturday, as I and several others, possibly everybody, are no longer getting e-mails. It was still interesting to find out how it was done, even though I never did crack it.