Never had that as a problem, deliberately use different floors when testing HomePlugs

But surely if sockets are on diff ring mains you cannot use homeplugs between them? Logically there cannot be any connectivity between them.

My house has its 2 ring mains as L & R of house (both up & down). Maybe yours is similar?

In reply to a post by XRaySpeX:

But surely if sockets are on diff ring mains you cannot use homeplugs between them? Logically there cannot be any connectivity between them.

As long as the MCBs protecting each individual circuit haven't tripped (fuses blown), both circuits are joined logically and electrically at the Consumer Unit (fuse box).

There will also be an electrical connection beyond the meter to other premises on the same electrical phase.
Dependent on distance, there may also be an undesirable logical connection to Homeplugs used in other premises on the same electrical phase.

Very true!