Porky Withers has been invaded by aliens. No, no, don't be alarmed. Do not scoop up your young 'uns and pack them into your station wagon and light out for the hills, casting anxious glances at the sky. Porky Withers hasn't really been invaded by aliens. He just thinks he has.

It is nothing to worry about, mind - it is just this time of year. There is something about the alleged festive season that, well, that drives Porky Withers mad. Actually there is something about the festive season that drives everyone mad, but it shows itself more obviously in Porky Withers.

Last year during the month of December Porky Withers imagined he had turned transparent. He was convinced that we could all look at him and be able to see what he had for breakfast that morning and supper the night before. Of course, we could tell what he had eaten for breakfast that morning and supper the night before, but we didn't need to look at him to do it. Porky Withers always has the same thing for breakfast and supper at this time of year - gin. At other times he might help down the gin with a piece of toast or perhaps an egg, if anyone gives him an egg, but at this time of year he sticks with what he knows.

Besides what he'd eaten, he was convinced that everyone could look inside and see what he was thinking and feeling. This made him edgy. "How are you, Porky?" someone would say, and he would snap back: "As if you don't know", in a hissy tone of voice.

We generally out-vote Sad Henry

Every Christmas, Porky has some new attack of strange ideas. Two years ago he introduced himself as Father Christmas's representative in southern Africa, with responsibilities for reporting on who has been naughty and who has been nice. "You don't think he looks into all your stories himself, do you?" he explained. "Out-sourcing, my friends. Everyone's doing it."

Each year we have an argument, down at the Chalk 'n Cue, about what to do about Porky Withers. Sad Henry is usually in favour of having him locked up, but his relations with Porky Withers have been strained ever since Porky announced he would be turning in an unfavourable report on Sad Henry. ("Why?" Sad Henry had demanded loudly. "What have I done?" But Porky Withers had just sighed and tutted and made a little mark on his clipboard and said: "I don't think you want me to say in front of all these people. Let's just leave it between you and the Big Guy, shall we?")

We generally out-vote Sad Henry, for three reasons: Porky Withers is our friend, sort of, plus it is Christmas, and anyway it's less effort to just do what we always do with Porky Withers, even when he isn't mad. We ignore him.

So this year Porky Withers thinks he has been invaded by aliens. It's all the fault of the Sci-Fi channel on DSTV. Porky Withers has been watching too much Sci-Fi channel lately. Given the time of year, someone would have stopped him, if anyone paid any attention to what Porky Withers does. One day he announced that aliens were speaking to him through his TV set. The satellite dish was picking up their conversations, he announced, and he was receiving them in his head. "What are they talking about?" we asked, for want of anything else to say.

"It's hard to tell," said Porky Withers, "because they speak in their, you know, alien language, which is more advanced than ours. It sounds like clicks and whistles."

It must have been a slow conversation day

"Clicks and whistles?" said Big Bob Plummer. "Like dolphins?"

"Exactly!" said Porky Withers, meaningfully tapping the side of his nose.

"But dolphins aren't more advanced than we are, Porky. They are less advanced. That is why they are the ones balancing balls on their nose and being rewarded with mackerel, not us."

"Well, that may change if the aliens have their way," said Porky Withers darkly.

It must have been a slow conversation day, down at the Chalk 'n Cue, because there followed a discussion about crop circles. Porky Withers informed us that crop circles were navigation aids to explorers from the sky.

"Really?" said Big Bob Plummer. "So what do they do if ever they visit a part of the world that, say, doesn't have any crops? Eh? What if they wanted to take a look at Zimbabwe, say? Would they become lost and fly around in circles until the lady alien persuades the guy alien to stop and ask directions? Eh?" Sometimes Big Bob Plummer thinks he is very funny.

Personally, I wonder how a spaceship that could navigate through the vast and trackless wastes of deep space to find a small, undistinguished planet tottering in the immense and three-dimensional nothingness would suddenly need the equivalent of cocktail-napkin doodlings on the surface of that planet in order to find its way around. But I realised that Porky Withers was not the man to ask.

"Doesn't Porky have family?" I asked Sad Henry, under my breath. "Maybe this wouldn't happen if he had somewhere to go for Christmas each year."

Sad Henry looked at me. "Porky Withers does have family to go to," he said. "How do you think all this started? He was sitting at the family Christmas table one year, looking around him, and he suddenly stood up and announced that John Lennon had been sending him messages at night.

Apparently John Lennon had been thinking it over, and he realised that he had it wrong: it is actually Porky Withers who is the walrus. And since that Christmas lunch, Porky Withers has never had to spend Christmas with his family."

"Do you mean... " I said slowly.

Sad Henry took a sip from my beer. "Porky Withers may be an idiot," he said, smacking his lips, "but when it comes to Christmas, he is no fool."