Extending a Query

I have an aggregate query that works fine, however I need to remove the hard-coded dates as follows;

SELECT Sum(IIf([vat],iif([weekendingdate] Between #04/01/2011# and #31/12/2099#, [takings]/1.2,iif([weekendingdate] BETWEEN #30/11/2008# and #01/01/2010#,[takings]/1.15,[takings]/1.175)),[takings])) AS Total, dates.WeekNo, [n-weeklytransactions].weekendingdate FROM departments INNER JOIN (([business units] INNER JOIN [n-weeklytransactions] ON [business units].buID = [n-weeklytransactions].businessUnit) INNER JOIN dates ON [n-weeklytransactions].ztotaldate = dates.Startdate) ON (departments.department = [n-weeklytransactions].department) AND (departments.bus_unit = [n-weeklytransactions].businessUnit) GROUP BY dates.WeekNo, [n-weeklytransactions].weekendingdate HAVING ((DatePart('yyyy',[n-weeklytransactions]![weekendingdate])=DatePart('yyyy',Now())-1)) ORDER BY dates.WeekNo;

This works fine
SELECT 1.2 as rate, Sum(IIf([vat],[takings]/rate,[takings])) AS Total, dates.WeekNo, [n-weeklytransactions].weekendingdate FROM departments INNER JOIN (([business units] INNER JOIN [n-weeklytransactions] ON [business units].buID = [n-weeklytransactions].businessUnit) INNER JOIN dates ON [n-weeklytransactions].ztotaldate = dates.Startdate) ON (departments.department = [n-weeklytransactions].department) AND (departments.bus_unit = [n-weeklytransactions].businessUnit) GROUP BY dates.WeekNo, [n-weeklytransactions].weekendingdate HAVING ((DatePart('yyyy',[n-weeklytransactions]![weekendingdate])=DatePart('yyyy',Now())-1)) ORDER BY dates.WeekNo;

What I need to do is to incorporate this
SELECT (select top 1 VAT_Rate
from VAT_Rates
where VAT_Rates.VAT_Date <= [n-weeklytransactions].ZTotalDate
order by VAT_Rates.VAT_Date desc) AS Rate, *
FROM [n-weeklytransactions] ;

I tried
SELECT Max((select VAT_Rate from VAT_Rates where VAT_Date <=ZTotalDate order by VAT_Rates.VAT_Date desc)) AS vrate, Sum(IIf([vat],[takings]/[Vrate],[takings])) AS Total, Month([n-weeklytransactions]![weekendingdate]) AS [Month], Year([n-weeklytransactions]![weekendingdate]) AS [Year]
FROM departments INNER JOIN (([business units] INNER JOIN [n-weeklytransactions] ON [business units].buID = [n-weeklytransactions].businessUnit) INNER JOIN dates ON [n-weeklytransactions].ztotaldate = dates.Startdate) ON (departments.department = [n-weeklytransactions].department) AND (departments.bus_unit = [n-weeklytransactions].businessUnit)
GROUP BY Month([n-weeklytransactions]![weekendingdate]), Year([n-weeklytransactions]![weekendingdate]), vrate
ORDER BY Month([n-weeklytransactions]![weekendingdate]);

but I get the following error "You tried to execut a query that does not include the specified expression 'ztotaldate' as part of an aggreate function"

Any ideas if this is possible?

Regards
Pete