Zero Sequence Suseptance = 0, Why?

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09. Overhead Transmission Lines and Underground Cables The Positive Sequence Model of Symmetrical Lines - Springer Transmission and Distribution of Electrical Power

Typically the positive sequence value is supplied by the cable manufacturer, the reason you don't get the negative value is because it's the same as the positive value. Zero sequence is a totally different story and is highly dependent upon the cable shape, whether it's multi- or single cored, shielded or not, in conduit or trenched, how it's grounded, soil conditions, etc., etc.

Remember that zero sequence currents are the ones that result from phase imbalances caused by asymmetries in loading, cabling, grounding conditions, and faults; therefore the zero sequence impedance is going to be dependent upon your local conditions.

Your best bet is to contact some cable companies and describe your configuration and let them guide you, unless you want to learn such esoterica as GMD (Geometric Mean Distance) and GMR (GM Radius).

I understand sequential componants and i also appreciate that the positive and negative sequences are the same in passive componants such as cables..... my difficulty is that data supplied for the suseptance contains only a positive sequence value, meaning that the mutual and series actual values cannot be computed, unless i am missing something?????

Sounds like you're trying to program your own load-flow/short-circuit analysis program, so you're probably well versed in matrix math. Unfortunately this editing system makes it very difficult to engage in any meaningful discussion of matrices. I suggest that you examine your assumptions regarding the computability of, and/or the existence of, mutual coupling values, they may be infinitely small.