A long-winded way of getting a seat for at least part of the journey is to get train to New Cross Gate, walk the 5 minutes to New Cross, and then all the space on the ELL you could ever want is yours! Incidentally, if you have a season ticket, then in the evening you can get the ELL to New Cross, walk to New Cross Gate (taking in the views) and pick up the train / ELL.

I realise that this does give rise to the question as to what value the New Cross destination gives (think we all know the answer to that one).