This is the Bittern we saw booming. Here is a description of that fateful moment:
"Crouching, neck bent back on itself, bill at a slight downwards angle and open but not widely. A wheezing inhalation followed by a slight, but visible, convulsion just as the 'whump' was produced, I think with a slight downwards flick of the bill. I noticed that this bittern produced a definite 'whump' compared to the 'blowing across a milk bottle' sound that all the others seemed to do - perhaps the less percussive sounds look less convulsive, but who knows. Pretty sure it boomed in response to another boom, but it ignored the same bittern booming both beforehand and afterwards, so not clear what provoked it into responding that time."