We knew that there are various problems with American carriers, and so we arrived at the Pan Am Express desk at National Airport in plenty of time. Even though we had reserved seats on the flight to New York, the flight was already overbooked. Pan Am apologises for this inconvenience, and has made reservations on the next TWA flight to Kennedy, upgraded to Business Class to compensate us. This wasn't really very comforting, because it still meant that we would have to change terminals at JFK, something we know to avoid.

It also meant changing terminals at Washington National. Pan Am has its offices at one end of the airport, TWA at the other end. This had already become apparent to us on our previous visits here. So we ran through the airport to the TWA terminal, aware that the later we were, the less chance we had of making the TWA flight, and our connecting flight to Mexico City. We only had a few days to spend there, and didn't want to waste it.

Arrival breathless at the TWA terminal revealed that we did not have Business Class reservations on the flight. In fact, we did not have any reservations at all on their flight to New York. The gate crew agreed, however, to put us on standby for the flight. What would have happened if we had not got onto that flight, I dread to think, but the three of us, along with about another six with similar problems, were put in single Coach Class seats dotted around the cabin.

Arrival at TWA's red terminal in New York posed another question: how to leave it. Signs marked "Exit" seemed absent, as did most TWA officers. When they heard that our onward flight was with somebody else, especially Pan Am, they lost interest. Eventually, we found the way out. A very small sign indicated the way to "Street". We caught the bus round to the Worldport, which is by contrast predominantly blue. Surely we would miss the onward flight. We found the gate, and checked in, and were surprised, by this stage, to discover that our preselected seats were still available.

Just behind us in the check-in queue (or line, as they say in America) were some passengers enquiring about the availability of standby on the flight. None - the flight was already full. I wondered whether it was overfull.

The time came when we should have been boarding, but there was no airliner in evidence. The prospective passengers started to become agitated, although Americans seem to take this sort of thing much more in their stride than Europeans. Probably they are more used to it.

I first realised something was wrong when I saw a check-in clerk take a boarding pass from a passenger and rip it up. This seemed most unusual behaviour. A few questions eventually elicited the information that the Airbus Industrie airliner allocated to this flight was not currently airworthy, and that another airliner was being brought to the gate.

However, the new airliner had a different configuration, with more seats in First and Clipper Classes. Clipper Class is the name Pan Am chooses to give their Business Class. A stream of passengers was called, and the ripping up of boarding passes was accompanied by the allocation of seats in a higher class. They could not have been entirely successful here, and I have two pieces of evidence to support this hypothesis.

First, two airliners of the same type naturally have the same floor area, so that if there are more seats in the premium classes, then more space will be used by them, leaving room for fewer passengers altogether. As the flight was full already, some passengers who had been issued with boarding passes already would be denied boarding.

Second, we had a view of a simple event from our Clipper Class seats, the memory of which will stay with me always. A Hispanic lady had been upgraded to Clipper Class, as had we, but when she and her baby arrived at the new seat, it was already occupied by a businessman, deep in conversation with the man, presumably a colleague, in the seat next to him. They wanted to upgrade him to First Class. He refused. Presumably he was discussing their joint strategy for the meeting in Mexico the following day, or something. The look of panic and dismay on the Pan Am staff faces was something to behold, when they realised that they had no option but to upgrade a Hispanic mother and baby with Economy Class tickets all the way to First Class.

As for us, we had an enjoyable but delayed journey to Mexico City, where we arrived just about four hours late. Despite that, it had been a surprisingly good flight. We travelled to our hotel, Galeria Plaza Hotel, found in the Zona Rosa in Mexico City, and before long we were settled in. We had a few busy days ahead of us, with sightseeing to do, particularly the Pyramids at Teoutihuacan, for which Mexico ought to be more famous. The museum of anthropology, the parks and the fountains are also under-rated. Don't breathe while in Mexico City, by the way. All you have been told about the pollution there is true.

Around Mexico City, we were delighted by the underground system. Not only was it always crowded, but to allow for problems of illiteracy, all the stations have pictograms displayed as well as their names. We searched in vain for a comprehensive map of the system as a souvenir.

As fans of Mexican food, we naturally ate out at every opportunity. Towards the end of the meal on the first evening, I was suddenly invited to join the dancers and the mariachi on stage to dance. This caused a certain amount of amusement to all present.

Mexico City is famous for street vendors. The most important phrase of Spanish is, of course, no gracias meaning "no thank you" and depending on the level of the vendor's insistence, we can move onto "I'm not interested" or the ultimate deterrent "We're not going to pay".

As well as vendors, services are also available. Most notably, shoeshines. Whether the man with the shiniest shoes in Mexico City is a shoeshine himself, or just very poor, is a matter for debate. After many refusals, I found one insisted on cleaning a very dusty pair of sandals even as I was walking along at a steady pace. When told we wouldn't pay, he stopped smartly, leaving me with a sandal with one small, shiny area and the rest still very dusty. Nobody else offered all the way back to the hotel.

Our friend described a shoeshine he had met on a previous trip to Mexico City. He had been very friendly, and given ample opportunity for him to practise his Spanish while having his shoes cleaned. The main problem was that the bill exceeded the price he had paid for the shoes in the first place.

However, one good buy in Mexico City is onyx. Muller's Onyx store sells beautiful onyx items, and there was no single item in the store we could not afford. Freight may have been a different story. Onyx tables which would be well suited to being communion tables were there, so were smaller tables, ornaments, flower pots, wishing wells and all manner of other things. We bought an onyx donkey about a foot high and a set of onyx liqueur glasses on a tray, while our friend bought a chess set and a set of glasses.