“Trainspotting” was Irvine Welsh’s peak moment, a quarter century ago, and what a moment it was: scabrous, funny, and Scottish as could be. Since then Welsh has reprised his four mismatched characters three more times, and now we have a fifth outing, “Dead Men’s Trousers.” It’s 2015 and Mark Renton is a manager of DJs, still travelling and using and swearing. Enter Franco Begbie, apparently reformed from his amok ways and a successful artist. Enter Sick Boy, still a no-good pimp. Enter Spud, reduced to beggardom. Each of them has his day in the sun, in a plot that careens without much purpose but with enough spark were it not for one sorry outcome: while I laughed all the way through “Trainspotting,” this novel is just not funny. Oh, a couple of chuckles emerged but most of the wild scenes were, simply, silly wild scenes. Welsh writes with huge flair, using his trademark Scottish vernacular (some readers will baulk at this, it takes some getting used to), but I turned the final page with a sigh. Vim and vigor but a rickety plot and mere chuckles.