When do I use "Partial Fuel-Up?"
 

I have a diesel, so I never let it get below 5% fuel remaining so it doesn't suck in all the rubbish.

If I fill at 15% remaining or so (therefore before the fuel warning light even comes on), is that classed as a partial refuel?

A partial fill is when you don't fill the tank. Fuel sold in the UK doesn't have rubbish in it and modern tanks don't rust and give sediment. Mind you, it is a good idea not to run dry on a diesel.

So you think, as I fill up with 10-20% remaining in the tank, I should choose Partial Fuel-Up each time, yep?

No. Only if you don't fill to the top. If you put a tenners worth in its a Partial. If you fill till tank is full it is not. It doesn't matter how much is in the tank to start with. I normally start with anything from a quarter tank to light on, but I always fill to full and the pump clicks off.

Partial is anytime you don't fill until the pump auto stops, or you can see fuel in the tube.