… has just been fired in Hyde Park? Why? From royal.gov.uk:

Gun salutes occur on the following Royal anniversaries: 

6 February (Accession Day) 

21 April (The Queen’s birthday) 

2 June (Coronation Day)

10 June (The Duke of Edinburgh’s birthday) 

The Queen’s official birthday (a Saturday in June)

14 November (The Prince of Wales’s birthday)

The State Opening of Parliament (usually November or December).

Gun salutes also occur when Parliament is prorogued by the Sovereign, on Royal births and when a visiting Head of State meets the Sovereign in London, Windsor or Edinburgh. 

Has Parliament been prorogued? Or perhaps someone is visiting.

Update: Looks like it’s for Jacob Zuma.