excellent detective work jakob! that's exactly what happened.
i imported the model from 3ds. the model was made outside of su in mm units. even though i selected mm for the 3ds importer, the model still arrived 1000 times too small. i knew (but had forgotten) that i should actually select m for the importer. so i scaled it up.
the invalid geometry thing doesn't surprise me...i had been changing the heights of raked walls which i find often causes issues, and had deliberately switched off "check model for problems"
thanks everyone 
i have to say that i have been away from sketchucation for a while, and it is so great to be back...the atmosphere here is noticeably friendly