Another 'lectric problem!

Lamp pops, low load, high resistance, fast flash.

Heavy load = low resistance thus high current.

Tbh the word "load" is weak, should be replaced by "wattage" for clarity

have discos got electronic flashers or bi metallic flashers?

I think they're bi-metallic, or a slightly more modern version of the same principal.

The relay has 4 spades iirc, power in, flash out, indicator display out, trailer display out.

It simply flashes like a normal flashed unit, but also decides what the wattage is and thus whether to display the trailer light aswell.

Cheers.