Thanks Daryl, that's a helpful link.
I was wondering about
Pitch and
Intonation so looking it up in Wiki was helpful:
Intonation, in music, is a musician's realization of pitch accuracy, or the pitch accuracy of a musical instrument. Intonation may be flat, sharp, or both, successively or simultaneously.
So its accuracy of pitch of the instrument at whatever fret.
The wording " musician's realization of pitch accuracy" is confusing to me as realisation can mean to achieve or to become aware of / understand... so does it mean the musicians perception/understanding of the instruments accuracy or the achievement of it by the instruments correct calibration? You see, I was born in the Dunce corner! Questions upon questions.
This was useful too....
Several factors affect fretted instrument intonation, including depth of the string slots in the nut, bridge saddle position, and the position of the frets themselves.
On fretted string instruments, pushing a string against a fret—aside from raising the string's pitch because it shortens the string—also causes a slight secondary raise in pitch because pushing the string increases its tension. If the instrument doesn't compensate for this with a slight increase in the distance from the bridge saddle to the fret, the note sounds sharp.
From Daryl's link:
Obviously it is necessary for the spacing of frets to be accurate if good intonation is to be achieved. Yet it is amazing how many guitars, especially luthier-made instruments, have inaccurately spaced frets. Whenever a guitar refuses to play in tune, fret spacing is the first thing that needs to be checked.
Assuming fret spacing is accurate, the second important variable affecting intonation is the stretching of the string which occurs when a note is fretted. This stretching increases the total length of the string, which increases the tension on the string, just as if we had tuned up with the tuner, and therefore causes the note to play sharp.
Things are not quite this simple, however. Each string behaves differently with regard to sharping tendency when fretted. There are three related rules which apply here: sharping from fretting is inversely proportional to pitch (the pitch rule); pitch is proportional to string tension (the tension rule), and; string tension is proportional to string mass (the string mass rule).
The pitch rule tells us that a guitar will display a global tendency to more sharpness as the open-string pitch goes down, and the Eb string does in fact go sharp more than the Et string. However, the tension rule and the string mass rule also come into play, and we see this especially when we compare the sharping behavior of the G and D strings. If we were to apply the pitch rule only to G and D, we would expect more sharping from D than from G. But D in fact sharps less than G. This is because of the metal windings on D which add mass. Even though D is lower in pitch than G, it has higher tension than G and therefore sharps less. If you want to test this, you can tune your monofilament G-string down to D; the string will now sharp more than when it was tuned to G.
Do I get a ribbon or something? 
