I have had a good look and I can't find the answer either here or on the Shelly help pages.
Before I try to install I want to check that I am going to do it right.
I have standard UK wiring for a one way light switch (It has two terminals, one is a common marked as COM or C. The common is for the live wire that supplies the input voltage to the switch. The other terminal is marked as L1 and is the output to the light fixture.)
I have no neutral.
The existing switch is a simple open/closed on/off rocker switch.
If I look at Fig 2 on the shelly picture it seems to show that I insert the Com and L1 in the right-hand side of the dimmer.
Live would connect to the 'middle' L input. The L1 (the output to the light fixture) would go into the O.
On the left-hand side, SW1, SW2 and L look like they just need continuity, but importantly no power?
If that is the case then I would take the existing live and L1 wires out of the switch and put them in the dimmer as described above.
Then I would need to put 2 short (unpowered) wires, one from the switch's Com to L, and the other from the switch's L1 input to SW1 or SW2. All this would do is create a continuity loop, but provide no power.
Sorry, I know that this is the most basic question, but does that seem right?
Thanks!