Yes I beleive that is correct for overrun brakes. If the weight of the trailer is greater than the car it says you must have couppled brakes not overrun which I assume is something like the air brakes that are on HGV trailers?
I took the part E test back in 2004. Its mostly common sence. I couldn't afford a £600 course so thought I just have a go at taking the test after a bit of practise with my dad's help. Test cost around £80 and I failed it the first time so it cost me £160 all in. I managed to find the examiners hand book on the net so I knew what standard was expected etc.
On the legal side, you are only exempt from paying duty on non-hydrocabon fuels such as veg oil. Engine oil would be subject to duty in the same way that you would have to pay duty if you run on heating oil.