boland:The part that's always blocked is kitchen drain only (kitchen sink + insinkerator + dish washer). So think an auger is fine.
Thanks all for the tips. Have bought some liquid drain cleaner, and an auger just in case it happens again.
All our houses have been on piles so drain pipe angles down from floor level to gully trap with no horizontal bits. Guessing that your 7 year old house is on a slab there's a fair chance that the pipe under the slab has minimal to no fall. Unless staff training includes running water long enough after using the insinkerator to flush the food mush out of the pipe it's likely some of it is sticking to the pipe.