Matching cells query

Hi guys

Having some issues here that I am hoping you can help me with.

I have 2 worksheets - main and match

In the main I have something like the following:

KeyID Company_name Address
123 ABC co 123 March Street
.....

There are 200 entries in the main work sheet.

In the match work sheet I have something like:

KeyID Company_name Address
123 ABC co 123 March Street

I need to be able to flag if the entry in the match work sheet is already in the main worksheet by using the unique keyID. This flag will be in the match worksheet.

I am using the following array formula:

{=IF(A3='[main.xls]Main File'!$A$8:$A$16,"y","n")} and was hoping that this would give me a Y in my flag column but this only shows a N.

If I just do a straight if statement then I get a yes:

=IF(A2='[main.xls]Main File'!$A$29,"y","n")

Any ideas what I need to do to get this to work????

Thanks