Running average formula (in same cell)

I need a formula that will show the average of a series of data:

In column E, a series of data is entered one at a time (E9:E13) which is calculated from the value of other cells.

E14 is to show the average of data entered between E9:E13. But it needs to show the average only for total number of figures entered (so if E9:E11 has been entered, E14 will show the average of E9:E11, similarly if E9:E12 has been entered, then B1 will show the average of E9:E12).

The default value of cells in row E is 0 which may complicate matters, as 0 is a valid entry.

I know I could have several cells for each average, but that is not what is needed, nor do I want to have to manually change the range that E14 refers to.

Can anyone help? I've attached the spreadsheet if it helps.