Access SQL - Convert Text to Number - sql

Question

I've had the same problem as described here: Access 2007 - Left Join to a query returns #Error instead of Null
for which, the solution seems to be; write a better calculated field column so that the error doesn't occur later down the line - described here: Access SQL - given two dates, return the dates of the previous period
However, my calculated column is really simple, only converting a text field to a number field (e.g. "003" -> "3");
RevisionNumber = CDbl([RevisionText])
Further to this, The process works in a left join further up the chain?!
I changed the calculated field to a constant as detailed in the comments on the answer. But this gave me a new error
"Each GROUP BY expression must contain at least one column that is not an outer reference"
Here's the comment:
In my test example, if I switched the calculated field for a constant (e.g. 3 or "Test") then this value did indeed appear for every chain, whether they appeared in the right hand side of the join or not. For a calculated field it returned an error. Only when it was a direct reference to a single field did it work properly. If I do solve it I'll post the workaround here. – Wilskt
So it must be in my SQL code??
SELECT QryDATATRIncompleteRequired.*,_
QryDATATRLatestCompleted.LevelTrained, CDbl([RevisionTrained]) AS [Revision Trained], _
QryDATATRLatestCompleted.TrainerSignOff, QryDATATRLatestCompleted.DateAwarded, _
QryDATATRLatestCompleted.ValidTo_
FROM QryDATATRIncompleteRequired LEFT JOIN QryDATATRLatestCompleted ON _
(QryDATATRIncompleteRequired.EmployeeID = QryDATATRLatestCompleted.EmployeeID) _
AND (QryDATATRIncompleteRequired.SubjectTitle = QryDATATRLatestCompleted.Subject);