Erratic display of events
Although I had to reformat the xml data to import it into Mediawiki as pages, it's content was exactly the same as that in the Simile-widgets timeline example data. This meant that the names of the event pages generated from the xml were the same as the Simile event names, complete with apostrophes and other punctuation marks. On the assumption that such marks in page names might be the problem, I removed them all. The problem remained.
I then set about removing the more obvious, potentially problematic characters in the event descriptions
( ) [ ] " ' / \ ? & etc. All were contained in the Simile data and the simile js seems to handle them OK but I thought it worth a go anyway. Still no joy.
I then removed the descriptions entirely from the timeline query statement and Bingo!! - all the events appeared.
Examining the 'description' texts more carefully by exporting to csv and importing into Notepad++ and Open Office calc, the only anomalies that stood out apart from many full stops and commas remaining in the text, were a carriage return character in two of the descriptions. Removing them cured the problem.
So, the lesson is that, if you want to include a text field in the timeline bubbles, make sure it does NOT contain any carriage returns.
It's worth pointing out that neither the characters mentioned above, nor the punctuation marks in page names causes a problem for timeline; it's just the carriage return character. I know this because all the experimentation that led to a solution was carried out on a test wiki. When the carriage return character was removed from the event pages on the main wiki, with all the other potential problems left intact, the problem was solved.