"stuart noble" wrote in message ...
Sounds like you have lousy sills which you should ideally replace
(particularly if they don't have a drip groove on the underside).
If you prefer to modify the existing, you need to get a flat surface,
preferably sloping away a fraction. Polyfilla Exterior filler is fairly easy
to use as a skim although I would use car body filler if you can face the
extra work. Then you need to get something genuinely waterproof on there.
Pliolite paint is good but any solvent based paint will be better than
conventional masonry paint, which is not at all waterproof IME.

Just to offer an alternative based on my own experience.

I was suffering something similar with 3 windows built in an extension
and went through some of the steps mentioned. It was only in some
weather circumstances that there was leakage.

What I did notice is that the window cills didn't have a drip - they
were sealed with silicon or something to the concrete cill but the
actual drip part of the wooden cill appeared to have been removed when
the windows were installed; the only reason I can think of for this is
to make it easier for applying the external render. I re-created the
drip with an add-on to the wooden cill and the problem has never
reappeared after 10 years.

Rob