Have you carried out a diagnostic to check for error codes?
Well, they might be. This problem has been coming and going. It would do this for a few weeks or even months
and then back to normal again. In 2012 I was told by the MOT tester that a new rule was about to be introduced and that it would fail the following year with this problem. (He was the one who advised me to take the bulb out!) This guy had to retire before my 2013 MOT, but somehow I was lucky, in both 2013 and 2014 not to have this problem during the test. Actually, in one of these two years I did the resetting (unplugging and plugging back in, following a video on YouTube) and it worked.
It is not working now... I actually took the seats off today to inspect the area, moved the cables around a bit, gave the connector another clean (vacuum+WD40) --- still flashing.
Regarding the error codes: my car is unfortunate to have the 16-pin connector already (the car is a 1995, N-plate) but not to work with the OBD II protocol (it started on Ford in 1996 or 1997). A garage guy I took the car to last year (there was a fault leading to bad emissions) has some code reader that he say is not common and is expensive. I guess it could be a glitch but I will only know if I take it to that guy again.
Thank you for your comments anyway, they are much appreciated.