I'm afraid that's right about black cab fares. If you are going to the airport there is the additional £5 drop off fee. Cheapest way, though also slowest, is Piccadilly line which goes through the centre of London. Heathrow express is a rip off!
No, sorry!
I used to enjoy looking at the golf course between the two runways at Don Muang, that I assume is still there. Reserved for the military, I was told.