IIRC it's one of the requirements in the Building Regs (They're online
- google for them including the term "odpm"). From memory (and assuming I understand the physical arrangement of Neil's building correctly) the logic is that it stops rain penetrating down the upstairs outside wall and into the inside below IYSWIM. Sort of rising damp but the other way up.
Neil, something you said worried me a bit, I hope I'm not too late. I think you really need to allow the new mortar to set properly in each short section before you start on the next. I know it will make the elapsed time for the job much longer but I can't see how the wall will stay up otherwise! I'll let others here with more knowledge and experience than me define "set" in this context, I find that mortar is still soft a day or so later and only really hardens properly over a week or so.