“In my opinion it should be lanced instantly. That’s the only reason I agreed to come off the garage roof.”

...or "how to stretch a bad pun just about as far as you can possibly take it". Whether or not you'll enjoy How To Get Ahead In Advertising depends solely on your view of a certain Richard E Grant. To some he's a comic genius, the best Doctor Who Mk 1 that never was (as shown in the Comic Relief spoof), and a spokesperson for students shopping in Threshers the world over. But he's the acting equivalent of Marmite - that "we demand the finest wines" line from Withnail And I will either be absolutely hilarious to you, or the aural equivalent of Uncle Monty jumping up and down on your testicles. There is no middle ground when it comes to his somewhat overripe acting style. And in How To Get Ahead In Advertising he's there, on-screen, acting his socks off for pretty much the whole of the running time.

But is How To Get Ahead In Advertising a horror film? Obstensibly, no - it's a satire on the advertising-led, consumer driven, "greed is good" 80s. But it's also a tale of dual identity and insanity, a-la Doctor Jekyll And Mister Hyde. And it's a film in which a man grows a boil on the side of his neck which turns into an evil second head. How much more "horror" do you need?

"Whatever it is - sell it!" spits Grant's character Bagley during a powerhouse presentation as the film opens. But his career as a top advertising executive is in danger of coming to a grinding halt, because he's developed a block over his latest campaign. Try as he might, Bagley just cannot think of anything practical to help sell a brand of pimple cream ("If it doesn't work on your blackheads, you can spread the fucker on toast!" is his latest, less-than-ideal, suggestion). Completely stumped, he tells his boss (Richard "I don't believe it!" Wilson) that the campaign is all-but finished, and leaves the city for a weekend in the country. But his lack of success is preying on his mind, and his behaviour becomes more and more erratic. Fellow travellers on the train are insulted, a dinner party is ruined, and when he wakes up the next morning he starts to destroy everything connected with advertising in his house. It's at this point his wife notices that he has developed a boil of his own, throbbing away on his neck. By the evening it's "the size of a fucking tomato", and then it starts speaking to him, prompting a remarkable spate of comedy running around.

A doctor is called and Bagley is sedated - the boil is bandaged up, but continues to spout advertising rhetoric from beneath the gauze. To escape the inane ramblings of what he has christened "the Mollock" (a combination of a mole and a bollock), Bagley takes to wearing a cardboard box on his head (the film's iconic image), much to his wife's chagrin.

Bagley's psychiatrist is called and told: "In my opinion it should be lanced instantly. That's the only reason I agreed to come off the garage roof." And when the bandages are removed, Bagley reacts in utter horror: "Oh my God! The bastard looks just like me!"

The psychiatrist reckons Bagley has turned his back on all the bad things in his life, and transferred them into his boil - "Your very own 'Big Brother'." With this, it looks like Bagley's going to get his wish and have the Mollock removed, but it's still growing, and as he waits for the operation in his hospital bed, it takes over. As his "real" head is swathed in bandages by its evil usurper, he cries out ""You're going to lance the wrong boil!"

With the Mollock in charge, Bagley can return to work. A new campaign is immediately invented, glamourising spots and pimples. Bagley is back at the top of his profession, but his wife is unhappy - he's obsessed with "blackheads and fucking, although not in that order".

The "old" Bagley is still there, but reduced to the form of a scar on his shoulder. When it tries to reassert itself, Bagley glues up its mouth. But his wife longs for the old, potty Bagley and watches a video he made before the lancing, where she can see Bagley, his head surrounded by cardboard, talking to a silent someone off-camera. As she watches, the new Bagley walks in and starts arguing with his old self (a tour-de-force scene if ever there was one). She has finally had enough and leaves him, Bagley's evil side now fully in control and ready to promote all kinds of rubbish.

To anyone who works in PR, there's very little in this film that could be called fanciful. The world is full of Bagleys wanting to sell us all kinds of rubbish, and that's what gives the film its power. Forget Withnail, this is Grant's finest hour, in a role which no-one else could play.