Help Needed With Format Of Query Expression

I'm using the following expression in the query grid to show the Month
and Year of the date field "TxtDate":

Week: Format([TxtDate],"ww yy",4)

I am grouping the results on this field.

Is there a different way that it can be formatted so that when I sort
the field it will sort by Week and Year e.g.

01 07
02 07
03 07

At the moment it is sorting as a text field:

01 07
10 07
11 07

I can't simply sort by the "TxtDate" field because it isn't part of an
aggregate function and if I Group on TxtDate or use Min or Max the
query gives undesired results.
.

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