Thanks to those who have responded so far.
I've previously used the stairs at Covent Garden, Elephant and Castle,Gloucester Road, Russel Square, Lambeth North, Borough and Regents Park. I'm pretty sure that at all these examples, there were signs at platform level instructing you not to use the stairs except in emergency.
As most people may or may not be aware this instruction seems to be relayed to passengers more at Covent Garden with notices and tannoy announcements advising passengers of the number of steps and instructing passengers to wait for the lift. I don't mind admitting that I'm one of many people who has ignored this.
I seem to remember that at both Elephant and Castle and Russel Square, the spiral staircase had a sign stating "Stairs to trains" and at Lambeth North there was a sign stating the number of steps contained within the staircase.
I remember an instance when I went to Covent Garden during a busy shopping day a few days after Christmas and the stairs were only available to passengers exiting the station. There were actually station staff enforcing this as well. From memory two of the lifts were transporting passengers upwards and the other two were transporting passengers downwards. I also remember an instance also around Christmas time when I travelled from Russel Square and there was a sign at platform level instructing passengers to use the lifts and that the stairs were for passengers exiting the station only. I never understood why the stairs were only available for passengers exiting at either example bearing in mind there were signs at both stations instructing you not to use the stairs to exit.