SORTED
Just done a lot of playing around - and noticed a script associated with the park light that flickers it on switch on. Found the script and opened it - and it refers to the name of the target as thelight!!! So when going into Play mode - the script is run (cos the park light object calls it) and its complaining it cannot find a scene object called thelight. When I put back the internal name in my modified park lights back to "thelight", all works as expected.
I guess - you got a park light object (modified or original) all the while it continues to call the script to flicker it on switch-on, you must leave the internal name alone else the script will error.
Lesson learnt - the hard way [sillyme]