Yes big gap between 08:21 and 08:42 (although these are the times at FH, HOP is two minutes later). And the Overground trains between these times are likely to get incredibly busy.

I understand that Canada Water platforms have been extended so that all doors will open on the 5 carriage trains as they are phased in.

I have no doubt that commuting at peak times is going to be hard work from Forest Hill, but it could be a lot worse. Personally I always arrive at work at 8am to avoid the crush (a habit I got into when trains were much more overcrowded before 2004, before that I would often find myself in the luggage car of a slam door train with 20-30 other people). And I'm looking forward to next year when I will switch from Southern to Overground and get to work five minutes quicker at no additional cost. I'm sure I'll get bored with standing all the way every day, but at least it will be on bright, air-conditioned trains.