Hi,

I'm planning to do a bit of exterior redecoration. We have a bay window which is a triumph of the victorian cowboys who built this place. Its foundations are (I think) two courses of bricks sitting on nice London clay. Consequently it's not very stable. The last time it cracked up it was shored up using helical bars. They're better than nothing, but there is still some movement, and things have cracked up a bit since the last repaint (about six years ago).

Anyway, as the options seem to be either underpinning it, or just filling the cracks every few years, I'm opting for the second. So the question is what's the best thing/technique to use to fill it. Here are two photos:
http://imageshack.us/g/254/imag0181g.jpg/
One shows the cracked rendered plinth. The other one shows a crack in the sill - it sounds pretty hollow underneath and I suspect when I give it a whack a large chunk of it will fall away leaving a big hole.

Cheers!

Martin