Ok - resolved this so I thought I'd share.
It looks like something to do with proxies/CORS. Whilst the map looked OK, various map.onload events hadn't attached.
My dev system on url http:\\mysys_dev referenced the API on http:\\mysys_live - when I copied the API over to the dev URL everything worked.
I haven't investigated the various options for controlling CORS yet, but in fact the situation is better for me, as I can have differnt versions of the API of dev and live