Actually It turns out that there is a problem with Identify. I think it occurs when the base map and the overlay layers are in different projections and the transformation between them involves a datum shift e.g. WKID 2157 to WKID 102113. Under these conditions the Identify function returns no results. If the base map and the overlay layers are in the same projection e.g WKID 2157 or WKID 29902 then the Identify works as expected.
Eamonnn