Paint's not really designed for this. I think some form of resin is used to repair cracks in concrete.

Google found this company. Looks like they might have the right sort of product for repairing larger cracks in concrete. Not sure if they have a DIY product though..

http://www.rockbond.co.uk

Hammie, just something that will stay white and not fade, bridge the gaps and yes gloss would be preferred, the masonry is not damp.
I should also add the masonry finish is not perfectly smooth so filler would look awful. Also it is extemely hard and nigh on impossible to rake out the cracks.
Thats why I am thinking of a thick paint type covering of some kind that would provide an overall new surface.
Thanks

Probably not textbook stuff, and it's always nice to see what your talking about, so forgive me if this sounds crap. I am assuming the cracks are a little bit bigger than hair cracks, so could you not mix up some exterior polyfilla, which is cement based and push it in with your finger, then wet finger and smooth it off. Then a couple of coats of exterior masonry paint. Being water based it will stay white.

I think that sounds worse than it is.

Actually I was going to suggest the same when I read it a few days ago, but I couldnt picture the window sill in my head if you know what I mean.

What hammy says will work, cause thinking back I have done it, not pushed into the cracks though, but sort of opened the cracks up wider, and filled, rubbed down, then filled again till its flush with the rest of the sill.

This might sound daft whats coming next, but again it will work and hold it the filler and prevent it from cracking.
Dilute sparingly a little PVA, whack that on the first filled rubbed down crack, when dry, fill again if its still proud, rub down again till flush, then whack again PVA on the crack.
When dry, undercoat and gloss as normal.

When I go into a job, and they have had cracks that have been filled but have opened up again, I use the PVA trick, and I open thier cracks wider, fill like I said above, and PVA each rubbed down layer of filler, what this does is, the PVA seals the filler, gives it like a hard eggshell feel, and gives the crack protection and helps to stop the crack from cracking again.

I have done this many times on inside jobs, and I even did it one time on window frames, and on a bungelow walls, and emuslioned afterwards and its has held, its worth a try.