[wideminded]

The bomb set off outside a Bali nightclub in 2002 killing over 200, including 88 Australians, was partly fueled by gas cylinders.

From the Wikipedia article on the Bali nightclub bombing:

"The car bomb was initially thought to have consisted of C4, a military grade plastic explosive which is difficult to obtain. However, on 21 October investigators at the scene disclosed that the main portion of the bomb consisted of ammonium nitrate, a fertilizer agent readily available in Indonesia."

There does not appear to have been any high explosive in the London bombs this week.

[naturalman1975]

Wikipedia doesn't tell the whole story. Yes, there was some form of high explosive used, but the vehicle was also packed with gas cylinders.

Plenty of references exist to the presence of gas cylinders - one example.

http://www.abc.net.au/lateline/stories/s703266.htm

It certainly doesn't seem to be identical to the bombs in London - as you say, we haven't heard of any high explosives in those bombs - but gas cylinders can certainly add to a devastating situation. Not being an expert, perhaps they don't explode easily - but fire on top of an explosion will do more than an explosion by itself - as seen in Bali.