Mail Merge Date Swapping Month and Day

I have a spreadsheet to mail-merge with a Word template document. The mail merge is triggered by VBA code. Some dates show incorrectly.

The Word-doc merge field is formatted thus: {MERGEFIELD DUE@"dd MMM yyyy hh:mm AM/PM"}

The Excel cell has the following date:

"01/07/2021 10:58:00 AM"

and has a custom format of "dd mmmm yyyy hh:mm AM/PM" so that it shows as

"01 July 2021 10:58 AM"

When merged this date comes out as: "07 January 2021 10:58 AM"

We are using the UK date format. The date\time on the PC is set as: Short Date: 24/06/21

The VBA code that triggers the mail merge is:

Set wdDoc = wdApp.Documents.Open(Filename:=StrMMDoc, AddToRecentFiles:=False, ReadOnly:=True, Visible:=False)
With wdDoc
    With .MailMerge
        .MainDocumentType = wdFormLetters
        .OpenDataSource Name:=StrMMSrc, ReadOnly:=True, AddToRecentFiles:=False, _
          LinkToSource:=False, Connection:="Provider=Microsoft.ACE.DDE.12.0;User ID=Admin;" & _
          "Data Source=StrMMSrc;Mode=Read;Extended Properties=""HDR=YES;IMEX=1"";", _
          SQLStatement:="SELECT * FROM `FTR$`"
        For i = 1 To .DataSource.RecordCount
            .Destination = wdSendToNewDocument
            .SuppressBlankLines = True
            With .DataSource

'..... etc

UPDATE: I have now tried several different date formats in both the spreadsheet and in the Word Template. Always with the same results.

I also added the following code to format the cell values:

Value = Format(DueDate, "dd MMMM yyyy hh:mm AM/PM")

Try this

{ IF{MAILMERGE DUE \ @"DD"} > 12 {MAILMERGE DUE \ @"DD/MM/YYYY"} {MAILMERGE DUE \ @"MM/DD/YYYY"}}