As promised, I have started to look at map layers that are not returning tiles. I will add to this list, cumulatively, as I plow through (nope, no need - finished!). Using,debug logging to file, I am examining the errors generated, then checking the logged URLs in a browser to see if anything revealing comes up. So far:

1) Yandex - returns tiles, but never for the correct coords - what system is it using?
2) Yahoo - fails, service unavailable (prolly true for all map types)
3) Virtual Earth - fails invalid request URL, URL seems malformed (true for all map types)
4) MapQuest Satell. - Service Unavailable (MapQuest Map OK!)

Debug logfile for the above available, if needed.

Not tested: Freemap.sk, Czech.

In addition: batch download around a route appears as if it is completing (and certainly takes it's time) but subsequent off-line driving quickly shows that many of the tiles are not available, at the zoom levels requested. I know this for certain but will need to run a separate test with debug logging enabled to see wots wot.