Hi David
Thanks very much for the image - that was really helpful.
I have a HP laserjet 3600 which was leaking magenta - and whilst trying to fix this problem - I must have ended up snapping off the part in question.
I managed to get the "microswtich" back into place with a pair of tweezers and the printer is now working again.
Still getting the magenta stripes - but I think that is down to the toner catridge.
Thanks again! Couldn't have sorted it without you.