This is from the Health & Safety Executive (HSE):

"There are, on average, three or four large gas escapes every year from gasholders, which could lead to a major incident if the gas found an ignition source. The main reason such an incident has not occurred for 70 years (though there have been fires) is because the gas has dispersed safely into the large, clear areas surrounding the holders. Development close to such sites (particularly of an intensive, multi-storey, nature) potentially increases both the number of ignition sources and of those people who could subsequently be affected by any incident. Recent incidents at other major hazard sites have shown that, although such events are rare, they should not be ignored when considering what developments to allow."