Hi again. A bit more information for you, Ben
I reran my Java program and pointed Switch at a folder of tiny files. It started fine, but this time when it reached file 10 (note, not 11, as before) it stalled again, displaying "Converting file 10" . However (and this might have been true for the larger files if I'd been patient enough) it then picked up, flagged a conversion error in the Switch window, and carried on to the next file. This was treated in the same way, and so on to the end of the folder. Each file (16K in length) after file 9 took about 5sec for Switch to decide it didn't like it.
I've investigated the Java "environment parameter" settings and have found that they're the options you might specify when launching a Java program - see here. Since I'm not currently specifying any, my exec(command) instruction is apparently launching Switch with the options that accompany Netbeans. They're multitudinous. I've played around with the one that seems to specify the amount of memory that Java allocates (eg -Xms1024m) but haven't observed any effect.
At this point, unless you've any ideas and can maybe tell me that Switch needs something special in its execution parameters, I think I'll accept I'm /way/ out of my depth and will just be grateful that things work when I start Switch manually first.
Best wishes, MJ