So from my understanding now:
You dont need a TV license if your TV is capable of receiving signal, you just need one if it actually is.
The only way they can tell if it is recieving is if you let them into your flat, which you are not obligated to do (or can they detect from the outside - if they can, in a block of uni flats how will they know it's yours?)
Also, you can watch TV on your laptop if its not power connected, but they obviously cant tell this.
And I could arrange to have an inpector come round, show him my TV linked up to an Xbox but not recieving any signal, then they will leave you alone for 4 years, in which time you can receieve TV...
How do they catch anyone???