Hi Jez,
Thanks for your message.  The widespread use of junction=circular seems to be fairly recent, or maybe the OSM wiki definition has shifted (as they do) to make them much more common.
If you compare 
https://wiki.openstreetmap.org/wiki/Tag:junction%3Dcircular
and 
https://wiki.openstreetmap.org/wiki/Tag:junction%3Droundabout
I think junction=circular, oneway=yes is correct here due to the (at least one, in this case two) traffic light controlled entry points on the N side, with junction=roundabout now only applying to 'normal' roundabouts with no right of way restrictions to the inner loop.  I think most common routers present them the same (eg OSMAnd will still say take the Nth exit at the roundabout) either way.
Cebderby (Clive)