Hi Robert, thank you.
Unfortunately that didn't work - as soon as I change identifyParams.layerOption from "all" to "visible", it just returns nothing (i.e. result returned successfully just there is zero result).
One thing though - the layers in the map services are turned off by default and are turned back to visible in the Flex viewer (so you can see them on map and they are checked/ticked in the TOC/LiveMapWidget). Not sure if this makes any difference.
Thanks.