Concatenate to create a filepath string and using indirect.ext to look up value

Hi

I want to sheet that looks up values from external workbooks several times.

I'd like to be able to have a cell for the filepath of the workbook and another for the filename.

I'd use CONCATENATE to generate a string which creates the filepath and cell reference.

Then pass that string into INDIRECT.EXT to return that value.

Eg.

D4 contains a filepath: P:\Projects\Project1\

E4 contains a filename: Workbook.xlsx

F4 contains the CONCATENATE function (I'd like to reference Cell C30 on Sheet called 'Data'): CONCATENATE("'",D4,"[",E4,"]Data'!$C$30")

INDIRECT.EXT(F4) returns a #REF!

Can anyone help?

thanks