Having tried everything, I wiped the BB this evening and tried again from scratch.
So far the mails coming in appear OK. But here is the interesting thing - the only difference that I can see is that the 'enable HTML mail' check is now NO. It was YES yesterday, but I did not change anything from leaving the 8100 to starting with the 8900.
I can only conclude that the 8100 is not affected by HTML to the same detail that the 8900 does. Can this be?
Not sure I am really liking the 8900 better than the old one, but working hard trying to!
Next question - is the 16Gb card OK on the 8900? The 8100 had trouble recognising anything above 4.