I am awfully fond of these early-seventies Ian Carmichael dramatisations of the Lord Peter novels, but just to be clear, first timers should know and accept the following:
1) These televesion plays were made thirty years ago on a shoe string budget. They bear little resemblence to our modern idea of a televsion show, being much more like televised theatre. *Slow* televised theatre.
2) Ian Carmichael's Lord Peter Wimsey is in some respects distinctly different from Dorothy Sayers' Lord Peter, he's much more physically & psychologically robust. Very hard to imagine him collapsing with nerves & he's certainly no chattering icicle; Carmichael's Wimsey is a solid, warm, commanding type.
If any of that bothers you, see the eighties-vintage Edward Petherbridge dramatisations of the Harriet Vane mysteries for higher production values, slightly faster pacing, and a true-to-character Wimsey. Much as I love those later efforts, I think you'll be missing out if you don't give these a try. The screen plays are very well adapted from the books & the acting is very good indeed.
Murder Must Advertise is the novel Sayers based on her own experience of years in an early advertising firm. After a mysterious death, Lord Peter arrives incognitio to try and puzzle out what is going on. I don't want to give too much away here and spoil anyone's fun, suffice it to say that this is a prime piece of entertainment & one to be savoured. My only quibble is with the perfectly *hideous* harlequin costume Lord Peter dons at one point. I mean, I know they had a miniscule budget, but this is so mesmerically unflattering that it leaves one wondering what on earth Carmichael had done to the wardrobe mistress to merit such retribution!