fyi
=ROUND(SUM('CTO < £250k'!AV116)/1000,0)+ROUND(SUM('CTO > £250k'!AV116)/1000,0)

is the same as
=ROUND('CTO < £250k'!AV116/1000,0)+ROUND('CTO > £250k'!AV116/1000,0) no need for sum in there