Buttressing and kentledge is the way to go, or rather the prefered method. If drilling is completely necessary, you can get the 4 hole plates from ADI that will allow you to hit the mortar joints only. They will do them made to measure if required.
You still have to go through the usual test and tie selection Fred, but the plate is a great solution. It was a good tip I got from a southerner, and used it quite successfully on a few occassions. It all needs to be decided before you start though as you will need permission.
Same as AOM but use chemical bonding - best to ask the conservation officer whats exceptable,i've dont crown buildings that allow positive ties & on the otherside National trust buildingsvthat only allow buttress work
So far, I have managed to avoid the chemical ties SP, but the jobs have been pretty basic as long as we could get light duty ties in. We also have struggled to convince some owners to allow us to drill but it has always worked a treat. The guy that put me onto it is actually very clever, he has come up with a few simple solutions to at first glance fairly complicated problems.