There are noises being made on FB that the "official" DD date has changed to 30th June/1st July. This info is based on some posts on the Lewisham Cyclists page stating that Southwark Cyclists are organising the return coaches for that weekend. Hopefully Jane OTP will be along son to clarify the situation.
My understanding is that there is a clash with the London Freecycle* event and that, since the same people (pretty much) are involved in the organisation/participation of both, it is necessary for one or other to move.
I have to say that my preference would be for the earlier date, on the grounds that there is about an hour's more daylight in late June than there is in late July. However, this preference could be academic as at some stage I will actually have to get on a bike and prove that I haven't forgotten how to ride one.
*as mentioned above by Rupert