So, my original formula is like this:

=AVERAGE(IF(('PIR Data'!Q:Q<>"")*('PIR Data'!T:T<>"")*('PIR Data'!T:T>=--"2012-01-02")*('PIR Data'!T:T<=--"2012-01-27")*('PIR Data'!F:F="Yes"),'PIR Data'!T:T-'PIR Data'!Q:Q))

Should it now be like this:

=If(ISERROR(AVERAGE(IF(('PIR Data'!Q:Q<>"")*('PIR Data'!T:T<>"")*('PIR Data'!T:T>=--"2012-01-02")*('PIR Data'!T:T<=--"2012-01-27")*('PIR Data'!F:F="Yes"),'PIR Data'!T:T-'PIR Data'!Q:Q))),"",AVERAGE(IF(('PIR Data'!Q:Q<>"")*('PIR Data'!T:T<>"")*('PIR Data'!T:T>=--"2012-01-02")*('PIR Data'!T:T<=--"2012-01-27")*('PIR Data'!F:F="Yes"),'PIR Data'!T:T-'PIR Data'!Q:Q)))