There’s not much confirmed online yet, but it looks like Charlton could be enjoying two Horn Fairs this year.
A ‘Horn Fayre’ in Charlton Park on Sunday July 27th:

And a ‘Horn Fair’ in Hornfair Park on Sunday August 3rd:

Neither are listed on the council’s events page yet, though the latter event is included on the Parksfest 2014 website (and seems to be supported by GLL, suggesting that there’s a link with the various leisure activities that they run in Hornfair Park). We’ve asked the council to clarify the situation via Twitter, and they’ve promised to get back to us:
If both are going ahead, it seems a little strange to be running similar events in adjacent parks on consecutive weekends, but perhaps this finally resolves the issue of how you spell ‘Horn Fair’: a different spelling for each park…We look forward to finding out more as information becomes available (on which subject, the organisers could learn a lot from the – volunteer – team behind this coming weekend’s Plumstead’s Make Merry). If you’ve got more information about either event, please get in touch!
UPDATE: After some confusion, the council Twitter feed has confirmed that both events are going ahead:
UPDATE 17th JUNE: Charlton House’s website has a bit more info on the Horn Fayre now: http://charlton-house.org/whats-on/event/42/horn-fayre