Since posting the above query a Club member more observant than me found that the instructions on the TCS "Installation Picture" page for the Spectrum SD45 was relevant to my problem.  It appears that the "works" of this loco is near enough identical to to D8-40c and, although labelled as not dcc ready is, in fact, ready.  So my question is answered.

I have a TCS m1 decoder for this loco but am puzzled as to fitting.  The only how-to I have been able to find refers to the standard non-ready loco which, I suppose, gives a clue but specific instructions would be better for me.  I'm a newbie to fitting "N" scale decoders. They are such tiny toys!