Electrics Question

Lighting query.

I have a single switch to add to an outside light. I have done so using a single cable to the switch which I know isn't usual but should be do-able.

WiringDiagrams.pdf (tlc-direct.co.uk)

Live goes down into the switch and comes back up and then joins the mains light Positive.
The neutral from mains to Light are joined together.
All 3 earths are joined together.

Issue : When the switch is off the light is dimm... when the switch is On the light works normally ?

What on earth is happening ?

PS not that it matters (the switch has a little light in it)

.

The cable required is a twin & earth to the outside light fitting, unless your using a conduit/trunking then it could be singles.
Theres no positive with AC just live and neutral plus an earth for protection.
The on off switch is placed in the live supply.
Simples

What type of bulb? (LEDs can stay on dim, due to neutral and earth bonding being poor, or because of the light in the switch allowing enough current to light an LED but not a old skool tungsten filament bulb)
or
If you remove the cables from the switch is it still dim?(at least one will be live so be careful, put in individual terminal blocks to be sure with the power off)
Can you meter the voltage at the light fitting?

And keep it safe, turn off at mains but always work as if its live.
Its possible to get a shock from a neutral if your body creates the return path for the electricity.