British Airways Boeing 747-436 G-CIVU operating BA242 Mexico City – London Heathrow stopped the climb at 12,000 ft this evening (4,000 ft above the 8,000 ft altitude airport at Mexico City) due to a cabin pressurisation issue, entered two holds to burn fuel whilst climbing to 15,000 ft before returning to Mexico City two hours after take off. After two hours on the ground the aircraft was able to depart again and succesfully completed the flight.