Thanks Pieter
I have experimented with grouping my map object changes between a single BeginUpdate and EndUpdate, and also tried grouping them into several batches, doing the updates one ofter the other.
Putting ALL changes between a single BeginUpdate and EndUpdate gave me the best performance, as you suggest.
The Map update slowed down considerably when I used several batches of updates. So my suggestion is that if anyone is making many object updates at once is to put them all between a single BeginUpdate and EndUpdate as Pieter suggests.
It works well now, my map refresh is working at 10 fps, with many hundred objects moving smoothly around the map. And I don't need to use a 'detect end of redraw'.
Jonathan