We have an old timberframed house which was completely renovated 40
years ago.

The side walls are rendered, and the end walls are brick.

I have a problem with the rendering on one corner in that the fillet
between the rendering and the brickwork has come away. See:

http://i36.tinypic.com/n5pqps.jpg and

http://i33.tinypic.com/23vj96q.jpg

My friendly neighbourhood jobbing builder pointed the crack with lime
mortar, but it pretty much all fell out over the winter.

The rendering is about an inch thick, and the piece adjacent to the
crack does sound hollow on tapping, but in spite of that seems quite
stable and unlikely to fall down for a while yet - I can't get any
movement when I try and pull it away from the wall. On the other
hand, I can't help wondering if it is imperceptible movement that
meant the lime mortat fell off, or if it was a bodged up job!

How would you tackle this? I presume lime mortar is the right thing
to use - if so, what proportions? I know PVA is used in these
situations - does one it paint onto the rendering/brickwork. If so,
does one let it dry before applying the mortar, and/or does one mix it
in with the mortar? Presumably it is important to wet throughly
before applying mortar even if PVA has been applied. Should I try and
filll the crack with anything else before applying mortar, and if so,
what. Is there any other kind of mortar that is more flexible and
more likely to stay put?

Sorry for all the questions, but this is not (yet) an area of
expertise I have developed yet!

TIA

Keith