Thanks. I'm very chuffed. I must say thank you to Neil for spending the time to look it over and verify that it is what I claimed it might be. I must say that I felt a bit lowly admitting that I had never seen a LDR before, but when Neil said nobody had, it made me feel better, although I am now on a mission to try and find out more and what the letters SDR and LDR mean.
Was - if it ever existed at all. Thought possibly to be some kind of post-war heater/radiator - see page 77 in Jim Dick's book. Or maybe is - there could well be one lurking out there somewhere, just waiting to be found - rather like your SDR was...