Queries :: Inserting With Duplicate Key Query

I have a load of data in an excel spread sheet that I'm trying to put into an access using Excel VBA (Jet/ACE SQL I think!?). I do this by constructing INSERT INTO strings and looping through my spread sheet. Let's say the primary key is associated with the field [Company] which holds the company names. A simplified version of what I was using (it all works great ) is below:

Code:
Set rs = New ADODB.Recordset
Set cn = New ADODB.Connection
cn.ConnectionString = GetConnectionString()
cn.Open
cn.Execute strSQL

where strSQL =

Code:
INSERT INTO tblTestMDB ([Company], [Date])
VALUES ('BP', '30/09/2012')

Unfortunately, I am having issues with users adding the same company twice. I'd like it to:

- update the existing company info if the company already exists
- add the company if it is not already there.

I'm trying to do it in two stages. First an update, then an add. The first part works great :

Code:
UPDATE tblTestMDB
SET [Date]='21/03/2015'
WHERE [Company]='BP';

The second part: (I have tried numerous variations on the syntax):

Code:
INSERT INTO tblTestMDB ([Company], [Date])
VALUES ('BP', '30/09/2012')
WHERE NOT EXISTS (SELECT * FROM tblTestMDB WHERE [Company] = 'BP');

I find the error messages rather cryptic but they include things like "Query input must contain at least one table or query." or "Missing semicolon ( ; ) at end of SQL statement." (<== No I'm not!?