Harry this is what I got out of that.
in cell C2 =AVERAGEIFS('Fuel Tanks'!$F21:$F51,'Fuel Tanks'!$A21:$A51,">="&'Fuel Cost'!C1,'Fuel Tanks'!$A21:$A51,"<"&'Fuel Cost'!D1)
D2 =IFERROR(AVERAGEIFS('Fuel Tanks'!$F21:$F51,'Fuel Tanks'!$A21:$A51,">="&'Fuel Cost'!D1,'Fuel Tanks'!$A21:$A51,"<"&'Fuel Cost'!E1),'Fuel Cost'!C2)
E2 =IFERROR(AVERAGEIFS('Fuel Tanks'!$F21:$F51,'Fuel Tanks'!$A21:$A51,">="&'Fuel Cost'!E1,'Fuel Tanks'!$A21:$A51,"<"&'Fuel Cost'!F1),'Fuel Cost'!D2)
and so on across the page, this returns last months value if no value is found. not the prettiest but it works.
Don