After some careful studying of the images, the rear derailleur is DEFINITELY a Positron 400.
http://www.disraeligears.co.uk/Site/..._(DG-300).html
A new derailleur would make your life much simpler if you intend to keep this bike.
And, JTG you are right, only the front shifter is there. Might as well get a set of new shifters...