On the opposite side of the road, on the same side as the entrance to Blackfriars Pier there came a sudden loud bang and a noticeable burst of orange flame leaping upwards. The bus arrived during the explosion (the driver didn't see or hear it as looking in the opposite direction and though it was very loud it probably wouldn't be inside a closed bus cab). I could see part of the orange 'mushroom cloud' receding through the window of the bus.
It wasn't clear precisely where it had come from and I asked the bus driver about it - he got out of his cab and we had a look over but couldn't see any more flames, there was a small amount of grey smoke drifting away but it seemed to be over. My first thought had been a car exploding but they don't tend to 'boom' neatly and then stop (limited experience of car fires but I've seen one in Blackheath and it was billowing black smoke and orange flame). I also thought about the bus explosion during the London bombings (I was in a bus about the same distance away when that happened and remembered a bang and plume of grey smoke behind me, but no flames*).
I tweeted about it then alerted the Police and British Gas via Twitter. While I didn't think it had anything to do with BG (these things apparently are often due to electrical problems) I suppose if you have gas pipes near a fire you might want to know about it!
Since then I've only seen traffic alerts saying that due to a fire in an underground chamber there's only one Northbound lane at Blackfriars and that this is also affecting the East-West lower road below it.
I wanted to add this post as a sort of data point as apparently pavement explosions are a bit more common than I might have thought. At least one of the explosions was caused by an "electrical fault in a link box chamber".
"THE chief executive of the country’s largest electricity network said he is kept awake at night worrying about the increasing number of pavement explosions."Glad I wasn't standing nearer to it!
"Figures from the Health and Safety Executive (HSE) show a sharp rise in the number of explosions in Westminster, with only three in 2010, increasing to seven in 2012 and eight in 2013, before going up to 13 in the first eight months of 2014. " - Power boss blames rain for pavement blasts as victim of Edgware Road fireball urges company to do more
Power cuts and rush hour delays in central London hit thousands as underground blaze which caused pavements to explode still rages after 24 hours (2 April 2015) Daily Mail
Cable firm boss blames rain for increase in pavement explosions in London (16 September 2014) Evening Standard
"Sixty four of the explosions in the capital have been reported to the Health and Safety Executive (HSE) already in 2014, an average of nearly two a week, surpassing the 51 recorded in the entire of 2013."
London exploding pavements surge prompts death fears (17 June 2013) BBC News
*I got off the bus and walked to work in Camden. I'd no idea at the time what had happened but there were worried people leaving Euston (my bus was just by the stop before Euston, not far from the British Medical Association offices where the explosion occurred). At the time we'd heard there were "problems on the underground" with electrical surges.