I would like to do a 3d countif across multiple sheets. I simply want to
count the cells containing 'D's. Something like =COUNTIF(Start:End!D43,"D")
would be ideal if it worked!

Unfortunately I see now that countif() doesn't work with a 3d reference. I
see some other answers here have used a =sumproduct(n(... function to get
round this, but didn't understand it.

Can anyone here help?