does a power line still work if the two sockets used are on different ring main I.e one upstairs ringmain and one downstairs electrical ringmain
It is likely to still work but at a significantly reduced speed if across two electrical circuits.
When I have tested my powerlines in the past (older HomePlug AV spec) the speed was reduced by 25% comparing the powerlines on the same ring main versus powerlines across ground floor and first floor rings. The effect on newer-spec powerline kit may not be quite so pronounced.
You can also get a reduction in speed if the power socket on the wall is a spur socket from a ring circuit rather than connected into the ring itself, even if the spur is on the same ring. Similar speed drops happen if you connect the powerline units into mains splitter adapters.