This due to failure of the sealant between the window mechanism and the door frame. Daubing sealant over the crack as suggested on many sites is not effective. A much better answer is to loosen the window mechanism and inject new sealant into the gap. 'Bog Standard' sealant is false economy. Superior products that cure in a wet environment are available from builders' merchants.
There is a join in the washer tubing, behind the wheel arch trim on the left side. It has a tendency to blow apart. The trim is not too difficult to remove, after swinging the rear seat cushions forward and vertical. I made sure that the joint would not fail for the third time by slipping some heat shrink tubing over it before pushing it back together. A hot air paint stripping gun will shrink the tubing nicely.
This is more prevention than cure. Some important parts are plastic and liable to fail under stress. It is well worth taking the trouble to free the rubber sealing trim from the glass when the car is washed, and to check this frequently. With a little used car and no garage the rubber binds itself to the glass, especially on the (for me) seldom opened passenger window.
While water alone may be used in the summer it is important to have a proper mix of screen wash during the winter. If the pipes freeze it is possible for the pump to blow open the joints warmed by the heat of the engine open on account of ice blockages further away. These joints are very hard to get at!