Only a good shaped pure hydratic lime plaster will work well in- and outside of a brick wall. There are many technical reasons, here are some of them:
1st The thermal expanding of lime mortar and bricks are very even (0,4-0,6 mm/m/100K), cement mortar will expand much more (~ 1,2) than the brick. Result: Cracks of the mortar, mostly seen outside, where is more thermal difference.
2nd The drying out capacity (after Cadlergues) of lime mortar is 0,25, brick 0,28, cement mortar 2,5. Result: The lime mortar will dry out damp ten times quicker than cement mortar. No storeing of water, no mold attack, no algues.
3rd The binding forces by drying out of the fresh mortar is low of lime mortar and very high of cement mortar. Result: No problems of binding from lime mortar on a 'weak' surface like an old wall, jointed normally with lime mortar.
4th The cement mortars have a lot of efflorescent alkalic salt deposits in it, which can make many problems. Only pure hydratic (not hydraulic!) lime mortar has no efflorescent salts in the receipt.
If you will know more details to this topic and can understand german, try this:
Masonry - a technical Guide
In english you will find info about mortar problems here:
Problems with lime mortars and others - a RILEM-lecture in the University of Paisley 1999
The problem is, that 'modern' craftsman are not more used to work with pure hydratic lime mortars, so they will recommend other things without respect of technical behaviour. Look for an old fashioned craftsman, there are many also in the UK. Perhaps ask the National Trust or Historic lime center, there they know the good guys (and give them my best wishes).
Good luck!
Konrad
