Vibration from front end when braking

[chaz]

I used to get a juddering from the front brake it would happen as I was comming to a stop say at a junction, it would start at about 15 mph and increase as the speed fell, the only thing I can put it down to is contamination on the discs/pads when washing the bike, I used to use wash and wax, now I use a tar & insect spray followed by mr sheen polish (WD40 to remove chain lube), if it's bad I use washing up liquid and water and try not get it on the discs, I'm still on original pads and discs after 37,700 miles and use front brake only for 95% of braking.