This formula appears to work with some columns of data yet when I change the
column field it enters the vale "o" when there is data to average...

I know this may sound crazy to some, but I am finding this really
complicated to solve.
=IF(COUNT((AVERAGE(IF('Summary Days'!$D$3:$D$1500="2",(IF('Summary
Days'!$K$3:$K$1500=37,(IF('Summary Days'!$P$6:$P$1500<0,'Summary
Days'!$P$3:$P$1500)))))))*ISNUMBER('Summary
Days'!$P$6:$P$1500)),(AVERAGE(IF('Summary Days'!$D$3:$D$1500="2",(IF('Summary
Days'!$K$3:$K$1500=37,(IF('Summary Days'!$P$6:$P$1500<0,'Summary
Days'!$P$3:$P$1500))))))),"0")

I know it isn't the tidiest formula... The columns I am looking at read
something similar to:

A B C
Priority Week Time Lapse
2 37 0.2
2a 37 0.0
2 38 0.6
3 37 3.2
2 39 0.0
2 37 0.0
etc

The Time Lapse column is calculated from other columns but the formula I
need is to find an average of the time lapse (for example, column C) if the
priority is 2 and the week is 37.

I have 1500 lines of data and a mix of all priority and weeks.

I also need the formula to populate the cell with a zero value if there
isn't any data above 0.0 (I found that other formulas were returning the
value DIV#/1!)

Please can someone help me find out where I am going wrong and explain it to
me so I can learn from my mistake?!
Many many thanks