As recently as 2015, a bomb that was dropped on Bermondsey in the Blitz was discovered in Southwark at Grange Walk and diffused.
Read the BBC News article here:
As new buildings are built with deeper foundations, previously undiscovered bombs are discovered and have to be diffused by the military. The website http://bombsight.org/ plots all the known bombs that fell on London.
When dropped by aircraft in an air raid, bombs would sometimes not explode either because that had not been manufactured correctly or they hit soft London clay which slowed the bomb gently enough that it did not trigger and explode.
If a bomb is found below the high water mark in the river, it is a job for the Royal Navy to defuse. If an old bomb is found anywhere else then it is passed to the Royal Logistical Corp, part of the British Army.