Thanks Foxy, that does appear to be what I need. I'm now struggling with the syntax to treat it as a single table for the purposes of a join.
This is what I've got:
SELECT * FROM _10003_catalogue as t1 JOIN details as t2 ON t1.id=t2.id
This is what I'm aiming at:
SELECT * FROM (_10003_catalogue UNION ALL _10004_catalogue) as t1 JOIN details as t2 ON t1.id=t2.id
I've tried putting parentheses in various places, and leaving them out, and just can't get it to work!