He went on to say that there's no way of telling in advance whether a brick is good or bad- you just have to build it into a wall and see how it lasts. I find this hard to believe: surely any decent craftsman has a feel for the raw materials of his trade?
Anyway, I don't mind the way the wall looks- crumbly brick is picturesque- but having the wrought iron gate fall out of the gatepost was inconvenient. So I had a long think (lasting about half a year), mixed up some mortar ("same consistency as cake mix." said Ailz) and glued everything back together. I didn't have a lot of confidence in the outcome- because I'd never laid a brick before- but I tested the work this morning and it was solid.