Basically, never use a mortar that's harder than the construction material.
This is a good explanation...
...although it's reference to what used to be called the old "restoration" mix is now considered heretical. I've come round to agreeing with that view. It's always worth taking a sample brick along to a lime supplier and asking what they recommend.
Brilliant link for this application, I'm fairly confident on my mix ATM if anything i might add another 1/2 white cement and lime just for some strength. In this application the brickwork is purely cladding the house so I'm not overly worried about its compressional strength, we used this mix to repair flint walls for years, people would crash in the same few spots 3-4 times a year, so after cleaning the same flints that many times we decided to make life easier and use a weaker mix with lots of lime in it. it gave the "flexibility" to withstand cracking, enough cement to stay standing, but very little adhesion so it came off the flints easily
The original walls were built with hydraulic lime and sharp sand and were incredibly flexible BUT were also incredibly sensitive to frost in the first few months of curing