I'd just like to add that by putting candidates through a session where they must demonstrate an NDB approach to land the examiner/assessor can get a good idea of a pilot's flying skills. A lot of NDB's dotted around the UK date back to the WW2 era. They are therefore unreliable and difficult to master when compared to modern, safer VOR or GPS signals especially for the approach to land - hence the reason for using them to test people!
What you must remember is that the receiving station, an ADF instrument points to the location of an NDB relative to you, and not strictly speaking, where 'to fly to' as is the case with other instruments.
So if winds and gusts are thrown in, you will no doubt be assuming a heading to correct for that wind (think visually for a minute). In this situation the needle on the ADF will be pointing somewhat to the left or right of your heading with the difference being Your Heading Minus the Relative Bearing. So you can see that the ADF instrument doesn't always give you a direction to turn to in order to fly over a beacon in addition to that you must factor in the wind into any NDB tacking. Therefore a candidate is also tested for his/her ability to judge and make the appropriate corrections/ignorations(
) for the wind which in all truth and reality can never be known with certainty.