I have the following query, which runs perfectly:

SELECT
[qry_BMO_District_Recap - RHS - Stage 2].Transit,
Format([qry_BMO_District_Recap - RHS - Stage 2].month,"mmmm yyyy") AS MonthOfYear,
[qry_BMO_District_Recap - RHS - Stage 2].DISTNAME AS District, [qry_BMO_District_Recap - RHS - Stage 2].Grouping_Name, [qry_BMO_District_Recap - RHS - Stage 2].Grouping_Priority,
IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Month_Count),0,[qry_BMO_District_Recap - RHS - Stage 2].Month_Count) AS M_NtoB_Count, IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Month_Amount),0,[qry_BMO_District_Recap - RHS - Stage 2].Month_Amount) AS M_NtoB_Amount,
IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Yearly_Count),0,[qry_BMO_District_Recap - RHS - Stage 2].Yearly_Count) AS Y_NtoB_Count, IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Yearly_Amount),0,[qry_BMO_District_Recap - RHS - Stage 2].Yearly_Amount) AS Y_NtoB_Amount,
IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Month_Count),0,[qry_BMO_District_Recap - LHS - Stage 2].Month_Count) AS M_BtoN_Count, IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Month_Amount),0,[qry_BMO_District_Recap - LHS - Stage 2].Month_Amount) AS M_BtoN_Amount,
IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Yearly_Count),0,[qry_BMO_District_Recap - LHS - Stage 2].Yearly_Count) AS Y_BtoN_Count, IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Yearly_Amount),0,[qry_BMO_District_Recap - LHS - Stage 2].Yearly_Amount) AS Y_BtoN_Amount,
[qry_BMO_District_Recap - RHS - Stage 2].AREANAME, [qry_BMO_District_Recap - RHS - Stage 2].Financial_Year
FROM [qry_BMO_District_Recap - RHS - Stage 2] LEFT JOIN [qry_BMO_District_Recap - LHS - Stage 2]
ON ([qry_BMO_District_Recap - RHS - Stage 2].AREANAME = [qry_BMO_District_Recap - LHS - Stage 2].AREANAME)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Month = [qry_BMO_District_Recap - LHS - Stage 2].Month)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Transit = [qry_BMO_District_Recap - LHS - Stage 2].Transit)
AND ([qry_BMO_District_Recap - RHS - Stage 2].DISTNAME = [qry_BMO_District_Recap - LHS - Stage 2].DISTNAME)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Grouping_Name = [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Name)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Grouping_Priority = [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Priority)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Financial_Year = [qry_BMO_District_Recap - LHS - Stage 2].Financial_Year);

When I try to reverse the above, as shown

SELECT
[qry_BMO_District_Recap - LHS - Stage 2].Transit,
Format([qry_BMO_District_Recap - LHS - Stage 2].month,"mmmm yyyy") AS MonthOfYear,
[qry_BMO_District_Recap - LHS - Stage 2].DISTNAME AS District, [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Name, [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Priority,
IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Month_Count),0,[qry_BMO_District_Recap - RHS - Stage 2].Month_Count) AS M_NtoB_Count, IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Month_Amount),0,[qry_BMO_District_Recap - RHS - Stage 2].Month_Amount) AS M_NtoB_Amount,
IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Yearly_Count),0,[qry_BMO_District_Recap - RHS - Stage 2].Yearly_Count) AS Y_NtoB_Count, IIf(IsNull([qry_BMO_District_Recap - RHS - Stage 2].Yearly_Amount),0,[qry_BMO_District_Recap - RHS - Stage 2].Yearly_Amount) AS Y_NtoB_Amount,
IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Month_Count),0,[qry_BMO_District_Recap - LHS - Stage 2].Month_Count) AS M_BtoN_Count, IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Month_Amount),0,[qry_BMO_District_Recap - LHS - Stage 2].Month_Amount) AS M_BtoN_Amount,
IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Yearly_Count),0,[qry_BMO_District_Recap - LHS - Stage 2].Yearly_Count) AS Y_BtoN_Count, IIf(IsNull([qry_BMO_District_Recap - LHS - Stage 2].Yearly_Amount),0,[qry_BMO_District_Recap - LHS - Stage 2].Yearly_Amount) AS Y_BtoN_Amount,
[qry_BMO_District_Recap - LHS - Stage 2].AREANAME, [qry_BMO_District_Recap - LHS - Stage 2].Financial_Year
FROM [qry_BMO_District_Recap - LHS - Stage 2] LEFT JOIN [qry_BMO_District_Recap - RHS - Stage 2]
ON ([qry_BMO_District_Recap - RHS - Stage 2].AREANAME = [qry_BMO_District_Recap - LHS - Stage 2].AREANAME)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Month = [qry_BMO_District_Recap - LHS - Stage 2].Month)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Transit = [qry_BMO_District_Recap - LHS - Stage 2].Transit)
AND ([qry_BMO_District_Recap - RHS - Stage 2].DISTNAME = [qry_BMO_District_Recap - LHS - Stage 2].DISTNAME)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Grouping_Name = [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Name)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Grouping_Priority = [qry_BMO_District_Recap - LHS - Stage 2].Grouping_Priority)
AND ([qry_BMO_District_Recap - RHS - Stage 2].Financial_Year = [qry_BMO_District_Recap - LHS - Stage 2].Financial_Year);

I get the error message "The field is too small to accept the amount of data you are trying to add. try inserting or pasting less data." I'm not attempting to run an insert or update query.

Access Help lives up to expectations by just repeating the error message, and the posts I can find on the forum which mention this error all seem to include something about a maximum of 254 characters in a field. None of mine are anywhere near that amount.

Does anyone have any idea what could be causing this?

You can get that error if you try and put decimal data into an integer field but you don't get it if you put integer data into a decimal field.

You can get that error if you try and put decimal data into an integer field but you don't get it if you put integer data into a decimal field.
Thanks for that.
When you talk about putting data in, what exactly does that mean? Displayng my ignorance, I thought a select query just looked up data.

You have a valid point, sorry.

I'd rather it had been invalid but found a solution.

Sorted! By a process of elimination - coupled with the fact that as soon as you ask someone else, you always work it out - I narrowed it down to the fact that the Transit field I was using to link the two queries was an integer in one and text in another (don't ask why, I just inherited it). Using Val() to perform the link appears to have done the trick. :)

Excellent! Thanks for posting back with your success.