How can I create a scotch egg with a runny yolk?
Timing and precision are key. First off, you need to boil your eggs for exactly 5 minutes, assuming they're large. They should be at room temperature before you start, and you should let them cool afterwards. This should result in a cooked white and a very runny yolk before you fry.

The oil you use to fry the Scotch eggs needs to be just the right temperature - too hot and the crust will brown before the sausage is cooked, too cool and the yolk will cook solid before the crust is browned. The oil needs to be 350F/180C; a cube of bread should take 1 minute to completely brown. 

If the temperature is right the Scotch egg should take about 5 minutes to brown evenly, and you should have properly-cooked sausage and a yolk that's still runny.