Thanks for that - in the end I went about it totally differently.
I have all the results of the ID task appear in a table - one table per layer - just like they used to in ArcIMS
At the end of each row (feature) I now have a button - this button takes the OBJECTID of that feature and the feature number and applies a filter to that layer. So the result is exactly what the users want - in those areas where there a 100s of features, they can ID the one they want, hit the button, and only that one is left - they then can print out just that one feature, without the others messing with the map.
Another button clears all these Isolations
The only pre-req is the existence of OBJECTID - which isn't a given - so much of our data are from other Esrti sites, that when they are imported the old OBJECTID is ignored and a new OBJECTID_1 is set up. But that can be resolved.