Doh! posted the following in the wrong thread:
OK, had a reply and it makes more sense now. The EPG data is pulled down from one of the random sites in the sources xml list. As Digi mentioned, there's a bug in the implementation of the plugin on the TM800 that means the randomizer doesn't work and as soon as the first in the list hits its bandwidth limit for the month, it all goes pear shaped.
You can get around this by trying the urls in the list manually until you find the first one still dishing it out, and then edit the sources.xml accordingly.
That's that sorted but unfortunately there's another issue. According to Rytec, despite getting the EPG info from a different site, the channels xml which is now pulled down automatically each time it's needed, still comes from the xmltvepg.be server. Therefore when that server is hammered (which is often) channels wont parse properly and so it bombs out at that stage.
This will have to be recoded by Milo. Personally I'd rather they also left the old way as an option and allow parsing against a channels list manually put in the epgimport folder for when there's downtime.
