Cheers, sounds like the problem... just found it, but have pushed the plunger in...
To set the brake light switch:-
Remove switch from pedal box.
Pull the plunger out as far as it will go! (one loud click)
Press the brake pedal down, and keep it there.
Replace brake light switch.
NOW let go of the brake pedal. (you should hear a series of little clicks)
Job done.
Check to see if the bracket that the brake switch is mounted on is not broken/loose.
While you are there press the clutch pedal with your hand if there is any movement from the clutch master cylinder then you need to consider changing the pedal box.
Cheers Trev.