It’s my understanding that if an ILS chart says intercept the Glide Slope at 3000ft ASL but the runway is at 500ft ASL I should intercept at 3500ft ASL, yes?
If that’s the case - and given that land masses don’t generally move much - why don’t they simply quote at the ASL height and be done with it?
Or have I got this all wrong? 
No, you intercept at whatever altitude the chart tells you.
If it says 3000’ and you’re at 3500’ then you’re above the localizer.
So the runway height is already calculated into the stated value? That makes more sense to me at least. Thanks for clearing that up.
Correct, no need to be doing math on final 
Just descend via ATC instructions and be at the the final fix by the specified altitude. In your example, 3000’.
Important rule is not still mentioned here in this Topic. Read that again, logically as ATC instruction so you’ll do math everytime that ATC require from you to do change of altitude? No. And now to important - AGL and ASL. In this case you need have set your altimeter at correct local pressure because then you will be again at wrong altitude although you have on your altimeter 3000ft. This is important here, also no one will report or required to do steps with AGL.