Have you adjusted the margins in the merge settings?
I have Peter. I have just retried moving the Top, Bottom, Left and Right margins (on the Multi Record Layout box) individually and all the same. The result is as if the Bottom and Left margins have a (say) 100 mm default. On the merged pages the Group All shows a neat border around all the cards which will then centre on the page perfectly. (There is no object hiding outside the original master throwing everything into an offset state!) The Page Setup AND the merge margins are all set to zero.
Did you press the preview button at any point?
Yes. And tried merges from both Preview Checked and Unchecked.
The preview can be buggy, and I think it can "infect" your template, so I'm going to suggest you make a new template file, then copy and paste your placeholders into that. Put them in the upper left corner.
If it's possible, make the placeholder group smaller than required to fill the page when duplicated 8 times with no space between, and set the top and left margins to position it correctly. In the merge options add the required spacing, leaving the top and left margins correct, but set the bottom and right to 0 (Data Merge can be odd about fitting when the margins are exactly right, too).
Finally take a leap of faith and merge without the preview.
Thanks. I will try this immediately and so long as I get no family interruptions will come back to you ASAP.
This has resolved my immediate issue of getting the cards fully on one page. But I have had 6 attempts.
Try (1) - roughed a card from scratch in the top left of the master and saved it. Result was a same as before the cards offset.
Try (2) - roughed a card from scratch in the bottom right of the master and saved it. Result very good. Cards as expected on the page.
Try (3) - returned to (1) and moved the card object to the bottom right and saved as a new file. Result as (1).
Try (4) - created new file document with copy/paste of the original "decent" artwork on bottom right of page and result perfect with margins!
Try (5) - created new file document with copy/paste of the original "decent" artwork on top left of page and result perfect with margins!
Try (6) - returned to (5) and used Preview. No problem with output!!!!
So what has this proved? Once there's a "glitch" it does seem to live on each time you rework/remerge despite altering margin settings etc. InDesign needs a new file to work with.
Perhaps the moral of the story is that try, try, and try again means start from scratch with a new file. At least I've not had to remove and reload CS6!
Peter - Many thanks for at least pointing me in a new direction and for your time spent in thinking it through. (BTW I used to be a full flight simulator fixer around the world and sometimes it'd take days sitting in training missions with pilots waiting for the glitch to occur so this time a couple of hours or so finding a fix has been very short.)
Better to wait for a glitch in the simulator than a real one in flight, I'd say.