Extract Information From Closed Workbook Representing Data From A Country

I'm having a problem with getting a result from another workbook. The purpose is to extract information from a closed workbook representing data from a country (ie US.xlsm, UK.xlsm) into the open one (summary). I want to count the number of results in a column which are above a certain amount (in this case 100).

So this is my coding for getting this result

=SUMPRODUCT(--(('C:address[US.xlsm]Worksheet name'!$E$2:$E$30000)>100))

Which works no problem for getting the number of results in the E column above 100. my issue is that I want to be able to change the name of the country in a cell, and then get the results automatically from the relevant workbook. I've been using CONCATENATE for these purposes and EVAL (from MOREFUNC)... and I get #REF!

So if I split up the above coding into two cells "=" and "SUMPRODUCT(--(('C:address[US.xlsm]Worksheet name'!$E$2:$E$30000)>100))" (just to keep it simple) and then use CONCATENATE to combine them in a cell. Using EVAL on that cell gives me #REF!

I've tested SUMPRODUCT with EVAL and CONCATENATE on formulae like the one above which do not link to other workbooks, and they seem fine. and SUMPRODUCT and linking to a workbook also works fine in the first example.

So the problem is somehow coming from the combination of SUMPRODUCT, EVAL and accessing an external sheet.