how are you defining a 'good bond'?

Good bond is when rubbed down very firmly with a wire brush 24hours
later there's no chance of dislodging the patch.

Update - we spent Friday solely on prepping the old spalled faces with
wire brushes (have now done about one-third of the wall). We also
pulled out all the less than satisfactory patches from Thursday. Most
of those still in place are brilliant, strong bond and good surface -
they look like pinky bricks mixed in with the reds.

The key factors seem to have been very through wire brushing of the
old faces and thorough wetting.

Our plan now is to complete the wire brushing of the damaged faces
(and reinspecting them until we're really confident we haven't got
sloppy anywhere), then limewash a few, then more thorough wetting,
patching - but with more experience of when to brush over (for the
surface texture) and when to wire brush (to clear off unwanted excess
mortar).

We've been using red brick dust to get the colour (and its pozzolanic
effect) - but the proportion has been 25% (i.e 1 part lime putty, 3
parts sharp sand, 1 part brick dust).

Can anyone tell me more about the possible effects of using such a
large proportion of brick dust?

Many thanks for all the helpful replies here.