The handbrake on switch can be bought from places like rimmer bros for about a tenner for a non genuine one. I got one for £7 from a scrap yard, did the trick for me but the light can also come on, as the others say, if the fluid level switch is faulty or dirty. Also mke sure the fluid is well topped up, sometimes even though the level is halfway between min and max the switch doe like it be nearer max!