Rocker switch & Electronic Relay switch circuit (240v)
I would use two SPDT relays inserted after the existing switch, like so:







simulate this circuit &ndash; Schematic created using CircuitLab

Hmmm - that takes care of conflicts between switch and remote commands, but might need something to prevent a conflict if both relays are operated at the same time.







simulate this circuit

Now, when RLY3 is operated, it removes power from RLY2's contacts - so a remote "Up" command will over-ride a remote "Down" command.

That still has a possible conflict between a local "Down" and remote "Up" commands, but I can't think of an easy cure at present - no doubt someone else will have a better solution.