So, I got a reply from IBM, linking me to a previous conversation topic:

*SPSS has five different methods for computation of percentiles (see the statistical algorithms for the EXAMINE procedure, available via Help>Algorithms). The method used in FREQUENCIES, and the default method in EXAMINE, is what's known as HAVERAGE, or the weighted average at X-sub-(w+1)*p, which we describe as the weighted average of X-sub-i and X-sub-i+1, where i is the integer part of (w+1)*p (w is the weighted case count, which would often be called N). This method can be confusing, as it can give results where the estimated percentile is higher than the case representing p% of the sample distribution. It is, however, the method yielding an unbiased estimate of a population percentile for p.*

A useful discussion of percentiles that features this version in its primary definition is provided in an online engineering statistics text provided by the U.S. National Institute on Standards and Technology at http://www.itl.nist.gov/div898/handbook ... prc262.htm . Another method that is widely used is AEMPIRICAL, which will give either one of the values in the data set or half way between two values. CTABLES and the Visual Bander both use the AEMPIRICAL method. It is the "third way" discussed on the NIST web site.
I've read over the literature and although by no means a statistics expert, it seems to me that the CTABLES approach is less accurate as it will be rounding the data at an earlier stage but it seems that either way is going to be an estimate of some degree, so it's a case of choosing the best to suit your needs (which I imagine will normally be the default HAVERAGE function)

If anyone can help interpret IBM's response for me into slightly more user-friendly language, that would be great? As I need to explain this away to a client and not sure if I could, based on the above!

Cheers,

Phil