Hi OldSteve
Sorry to take so long to get back to you. I had a chance to sit down this evening and test whether a simple sketch would compile using analogReference(INTERNAL2V56).
void setup() {
analogReference(INTERNAL2V56);
}
void loop() {
}
Unfortunately I get the following error.
exit status 1
'INTERNAL2V56' was not declared in this scope
If I however change the sketch.
void setup() {
analogReference(INTERNAL);
}
void loop() {
}
My code compiles. 2.56V is my preference reference. 
I've done some further searching and found the following thread on the Arduino forum.
If you check out specifically post #6 there is a odd suggestion involving defining a constant. I'm not quite sure what's happening here although if I copy and paste the code it does compile? 
#define INTERNAL2V56_NO_CAP (6)
void setup( void )
{
analogReference( INTERNAL2V56_NO_CAP );
}
What does the bracketed six represent?
Cheers
Jase 
PS I'm not sure what 'Core' means? I've tried reading up on this and have also spoken to a retired Electrical Engineering lecturer and the best I've come up with is it's the guts of the chip???