I would go with most of the above - you need to clean before you can sterilise. Pour in hot water, a touch of washing up liquid, and something hard that you can shake around to dislodge the deposits. Trouble with demijohns is that they are too big to use a bottle brush on, so go with small pebbles (pea shingle). Once you have got the surface muck off you can then sterilise. For further use, please clean them as they get emptied - it's much easier than having to deal with dried on muck.