Its really is more of a black art than a science
The postcode checker above won't help much in the cities
If you're in the middle of a lot of tall buildings, the signals will be bouncing around all over the place & coming in from different angles & at slightly different times.
Most decent aerials [even some indoor ones] have decent rejection of unwanted signals, the trick is to align it on a good feed & digital makes this a right ball ache at times.
Given the cost & inconvenience of have a proper antenna installed & your requirement for a short term solution, an indoor aerial seems to be the way to go [although it may not work]
Have a look around you & see which way other aerials are pointing & start there
Tune the TV & see what you get – If you get nothing, turn it 45 degrees & tune again
Half this again if you need to so the the aerial is between the two points you've already tried
If all that fails, try it 45 degrees in the opposite direction from where you started etc. etc.
You must re-tune after each move
Have fun 🙂
If you have a SE facing window, one of these is easier to align
http://www.aldi.co.uk/uk/html/offers/special_buys3_13988.htm