Apologies just sorted it.

For info the problems appears to have been at the end of the whole
formula...

=(SUM(IF('Key Delivery working sheet'!$B$4:$B$59998=$C14,IF('Key
Delivery working sheet'!$J$4:$J$59998="D",OFFSET('Key Delivery working
sheet'!$B$4,0,VLOOKUP(Summary!$F$12,'Key Delivery working sheet'!$EI
$3:$EP$19,7,FALSE),65000),0))))

It was the 65000 that was causing the problem as it was looking at a
disproportionate number of cells...

When changed to:

=(SUM(IF('Key Delivery working sheet'!$B$4:$B$59998=$C14,IF('Key
Delivery working sheet'!$J$4:$J$59998="D",OFFSET('Key Delivery working
sheet'!$B$4,0,VLOOKUP(Summary!$F$12,'Key Delivery working sheet'!$EI
$3:$EP$19,7,FALSE),59994),0))))

i.e 65000 to 59994 the number between B4 & B59998 it is now working
like a dream.

Many thanks all the same!!!

Worzell