Still, I'd like to know how to identify the objects referred to by the bracketed numbers, so I can tidy up the few "scrap outline intersects itself" errors that I get.
"Scrap outline intersects itself" warning gives the scrap reference, so look at the walls in that scrap. In some older thread (or wiki perhaps?) there are examples of some weird self-intersections. Anyway, after some fixes in PDF processing this warning is not so critical as it used to be and the map usually looks fine. For real errors it is not easy to give a convenient reference, because MetaPost does not offer any exception catching facilities. Currently you can look at the line starting with 'l.<number>' (in your case l.18855) -- the number is the line number in the metapost file data.mp where the error occured. In this file you can find the name of problematic scrap (look for current_scrap definition preceding the line with error). The file data.mp is located in temporary directory thTMPDIR, which is created if therion is run with -d option. Bracketed numbers are MetaPost figures (the number is displayed after the figure is processed). Each figure contains some part of some scrap. In data.mp each figure starts with "beginfig(<number>)". Martin P.S. The most common source of errors are lines with zero-length segments. We need to fix therion so that it would not export them for metapost -- than the errors could be much less frequent.