Personally for the long term I would remove one side of the bin and empty the bin completely, yes this will be quite hard work but in the long term it will be worth it.
Once emptied I would form a partition to split the bin in two.
Then I would sort out the stuff you have removed into usable material and put this on your plot, the remainder I would put into one side of the newly partitioned bin and leave this till next year (2018). The remaining side would be filled with material that will accumulate this year (2017) this would be left to rot down to be used in 2019.
In subsequent years you would alternate filling the bins in a similar manner.
If you are a glutton for punishment you could form an extension on your bin to make it into a three compartment bin and every six months empty the oldest material into the extension which is effectively the 'turning' process Johnny was referring to.
The key to good composting is to form layers of different compost materials, there is a list of these in this article;
http://www.thegardenersalmanac.co.uk/Data/Composting/Composting.htm