Automatically number spreadsheet

Hi,

I have an equipment list that needs to have a fixed Ref number inserted into the 3rd Column c whenever there is a quantity entered into the 13th Column M.

This I understand is quite easy to achieve with a formula, however the tricky part is when I need to add a row.

The new Row,( inserted below the active cell needs to be numbered with the Same # as the Row above + a 2 digit decimal point . For Example. I need to Insert a Row between Row 10 and Row 11. The Row is inserted, the formulas are copied down and the row is numbered 10.01. If I need to add a 2nd Row, that row is numbered 10.02. The row after that is still Row 11.

SO the Ref Column would look like this :
10
10.01
10.02
11

These numbers would not change once they have been entered and if possible they should be locked.

Thanks for any help and or advice

DMWllC