Thanks Antonius, but it's customer data so I can't share it.
I've just tried importing into Creo2 but the parts still arrive with tiny gaps. I miss the old (2001?) 'Zip Gaps' function - it wasn't perfect but it was a darn sight easier to do something with than the current IDD.
Radovan, the default accuracy option seemed to be External - I've tried that and Automatic, with neither producing a solidified component.
Oh, and it takes about half an hour to import the STEP each time (flat out on one CPU core). 