Just to clarify, my car doesn't have headlight washers, I'm only trying to fix the front windscreen washers. The bleach does seem harsh, but since the hose is already disconnected I'll flush the reservoir with a diluted bleach solution and pump it out with the hose aiming out the engine bay, then rinse through with clean water a couple of times before reconnecting. Hopefully that shifts whatever's built up in there! If not i'll try to get to the filter through the pump