Tristan appears from the second episode of the first series (which is a corker). I think he's possibly not in some of the later series (I know there's a gap, and then they revived it a wee bit some years later, but I forget when/whether there was a cast change: Calum Buchanan replaces him, maybe?). He does keep nipping off to Edinburgh to do tarts exams, but fortunately he keeps coming back again, in the early series at least.