I have an downstairs outside bay window ledge which is cracking up, as
they do. It's the common thick and deep concrete wrap-around thing
which is cast on site. The cracks range from hairline to a couple of
mm wide and seem to go right through, although the ledge doesn't seem
to be about to fall apart and I want to fix it before it does.

Googling this group suggests that PVA as a good filler/glue for cement
cracks, but the tin doesn't give any instructions for this
application. In fact it only talks about using it as a sealer, but
this (UniBond waterproof PVA) was the only "outside use" tin I could
find in Homebase.

What dilution should I use?
Should I add cement to the mix as the tin says? Or will this make it
flow less freely?
Do I pour it in a bit at a time allowing it to set between pours, or
just fill 'em up in one go?

Or should I use something else entirely?

Previous attempts with outside Polyfilla haven't worked for any length
of time.

Chris