Does anyone know how worn your front pads need to be before the warning light come on?

Reason I ask is that my warning light is not on, but I have just looked at my from pads and they are nearly metal to metal! I can't see exactly without taking the wheel off - which I haven't got time to do now, but I would have thought the warning light would have shown its face by now!

you'll probably find it tucked up out of the way of the disc then , I think they wear down and activate the warning when about 4mm of pad left

No, its in place, it has to be connected as its obviously a closed / continuous loop circuit?

I'm guessing there is only 20% of pad left - about 2mm

Had these out today and the sensor was nowhere near the disc. I would say my pads were only 50% worn.
I will measure them tomorrow and report back

Sounds like you've got a sticky slider / pin / piston and it's causing uneven ware to your pads. Had it happen on mine, wore the pads down the the metal on the side that didn't have the sensor, while the sensor side had plenty of meat left on the pads Didn't mess around, got new calipers and pins, as I didn't know if it was the piston sticking or the pin, and it's an old BMW tractor

And now your going to say that you've got Brembo brakes In which case it will be sticky pistons, and a rebuild would be advised  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!