Thanks for those tips Anna - I think we're going to have a change of
tactics today - I'm going to concentrate solely on prepping the
spalled bricks and limewashing the damaged area.

Coverage was generally 15-25mm (i.e.fairly deep spalling) and North
facing so not getting the sun - but it's 100's of bricks. Having the
whole face lime-rendered might be necessary at some point.

Overnight the patches have taken a good set (the ones rubbed down with
a wire brush late last night have taken the most natural, blended-in
appearance, ones brushed down earlier - less so) - but probably only
50-60% have a good bond.

I think the change of tactics today will reduce any tendency to "get
the mortar on" without sufficiently thorough prep.