I worked this out with the help of support. I had no idea the phones could copy phone directories to each other directly on the internal dect network. I found this option in another phones directory options and managed to copy the list to the new handset ;)
As the title says, using this particular combination of base station (admittedly an old one) with a CL390HX handset works in general but the "transfer" of a contacts file to the handset fails instantly with the message 'copying cancelled' appearing on the handset as soon as the copy is initiated in the A580IP admin web page.
The official Gigaset response is, well its an old base station, we haven't tested this combo and it may well not work.
However I am somewhat surprised by this as I thought that this kind of function would have remained a core element they'd want to keep always compatible. The Gigaset range has done so incredibly well at keeping new handsets compatible with old base stations but it seems that at last I may be expecting too much.
I'm writing this post just in case anyone has any tips or ideas on copying my contacts directory to the new handset ?