In short.
The big orange and white radar is a medium range 23cm radar It is known as the TSF and is a test facility for all the en-route HSA's around the country. i.e. major parts are tested in it before being classed as servicable. Thats why it is sometimes turning and somtimes not.
The one that abuts the airfield is a 10cm radar mainly used for TC purposes whilst controlling a/c's in and out of Gatwick. That's why it should be turniung all the time.
Hope this helps.