IT WAS something of a sick joke. The inspector at the bus garage door said, 'Happy bloody Christmas,' and handed me and my mate a Roneoed memo. This was Chalk Farm at the start of the Christmas season and we were the driver and conductor of the oversubscribed 68 bus that trundles south over the Thames to Croydon. 'This is your Christmas card,' he said. 'P45?' asked my mate. 'Watch it,' said the inspector, as inspectors do, and walked off.

What did our official London Transport Christmas card show? A robin on a log? A view of St Paul's in the snow? A stretch of bland Home Counties countryside depicted in the winter gloaming? Nothing so refined. What it said was, 'Subject: Xmas Vomits,' and continued: 'Due to the increased number of vomits during the Christmas season, crews will be expected to help in the cleaning of buses,' or words to that effect. It brought a lump to our throats.

Inspectors know from bitter experience that vomit is to Christmas as chocolate eggs are to Easter. They know that at Christmas, London buses are transformed from cheery red double-deckers to mobile vomitoria.

Did passengers perform the Christmas we got our Roneoed card? As if on cue. From Chalk Farm to South Croydon, gullets opened wide to eject 'Xmas vomits'. It was as if the whole population of London has been dosed with a powerful purgative. Perhaps it had, the medicine being alcohol-aided and abetted by the blotting-paper food served at most office parties.

Late at night, our Routemaster would gargle its way south from Chalk Farm, while I soaked in the fairy-tale views of the city from the snug cab and passengers giggled, guffawed, went green and puked. They chundered at Camden Town, cried Hughie at Herne Hill, threw Technicolor yawns at Thornton Heath and, we supposed, when they got home they would be 'driving the porcelain bus' for much of the night.

The transformation of cheap wine, crisps and sausage rolls into what the Scots lurching perilously on the rear platform called 'pavement pizzas' was almost miraculous. While herald angels sang - courtesy of Sally Army bands trumpeting along the route - and we dreamt of royal David's city, secretaries and clerks, junior managers and the odd confused solicitor (should have got off at Camberwell but ended up in Croydon at half past midnight), turned pizza and wine into a warm, slithery carpet of acrid sick.

Viewed from the cab of a double- decker, this process was not really so extraordinary; from here you watched daily as Londoners strewed sweet wrappers, spent chewing gum, cigarette butts, fast-food containers and lager cans along the city streets. The fortnight before Christmas was different only in that they generously added the contents of stomachs working overtime.

Xmas vomits are proof that Christmas is really still what it was before the birth of Christ: Saturnalia, a time of bacchanalian revels and bulimic guts. The image we have of Romans vomiting up plates of guinea fowl, to make room for a roast swan stuffed with bees' testicles basted in honey, is not so very far from that of Londoners chucking up quarter-pounders and lager in preparation for the next day's office party.

Still, the 68 bus got incontinent London safely home every night and took it back to work again the next morning smelling not of Christmas stocking scent but of powerful disinfectant and Milk of Magnesia. This, sadly, was no cure for the heaving sickness of the night before, for the less-than-subtle combination of hangovers, disinfectant, diesel oil and the ship-like sway of the bus as it wound round Norwood in the south and snaked through Camden in the north brought up what had failed to come up the night before.

Since then I have avoided taking the bus in the days leading up to Christmas. Instead I spend the holiday in Muslim countries and I open Christmas cards as if fearing the worst.

(Photograph omitted)