Moving average calculation is relying on order for reference, and for this purpose you can use Currentrow() or Rank another field, such as [Date].

Previous Close = SUBSET_SUM([Close], Subset([Row],[Row]-1,"="))
# establishing the previous value, with [Row] reference

Quick formula for n=14:
SUBSET_SUM([Close],
Subset2([Row],[Row],[Row],[Row]-13,"<=",">=")
)/14

It is better to include a safeguard IF function and Intersection function
IF([Row]<14,
null,
SUBSET_SUM([Close],
INTERSECTION(SUBSET([Row],[Row],"<="),SUBSET([Row],[Row]-13,">=")))
/14
)
Here is a link for the Subset functions: http://www.visokio.com/kb/subset-functions
There are several other forum posts offering examples how to calculate moving averages:
http://forums.visokio.com/discussion/comment/6201/#Comment_6201
http://forums.visokio.com/discussion/1522
http://forums.visokio.com/discussion/comment/4522/#Comment_4522

HI Paola,
Thank you, However I'm struggling with the next bit of logic, the K18 formula. i.e. it's to Take ((K17*13)+gain)/14. I see that for row 17, 18 works but not further than that.

That is a self-referencing formula, where result of each cell is product of result in the cell above (same field). It is possible to achieve this using a scripting formula:
SCRIPT(`
var a0 = formula("CELL([A0],"+currentrow+")");
var n = formula("CELL([i],"+currentrow+")");
var val;
var str;
if (a0==null || n==null) {
val = 0;
str = "";
} else {
var val = Math.pow(13/14, n)*a0;
var str = "13/14^"+n+"*"+a0;
for (var row = 1; row <= allrecordcount; row++) {
var iRow = formula("CELL([I],"+row+")");
var gRow = formula("CELL([G],"+row+")");
if (iRow!=null && iRow <= n) {
val += Math.pow((13/14), n-iRow)*gRow/14;
str += " + (1/14)^"+(n-iRow+1)+"*"+gRow;
}
}
}
val;
`, "currentrow", CURRENTROW(), "allrecordcount", ALLRECORDCOUNT())

[AO] is the first value, [I] is row number field and [G] is gain.
File is attached.