Excel 2007 : Argument substitution in SUM

Good Afternoon Everyone,

First Happy Holidays !
I am having a problem substituting an argument within the SUM function.
I have a basic spreadsheet which I use to account for my time throughout the year. The WorkBook starts as a single template which is copied to each Week (sheet) as it is needed culminating in 50+ sheets by years end.
One function that is handy is to be able to look at the cumulative hrs spent doing whatever. I had been using a system where the 52wks are sandwhiched between 2 sheets : 1 called "First" and the other called "Last", then any summation was simply : =SUM(First:Last!A1) This has worked well, but I would like to be able to change the
function so that it only reflects the time accumulated up to the current week
ie. =SUM(First:Week5!A1) this way as you reach the end of the year and look back to any given week the time calculated is only the time to that week.

I have been able to extract the current week from the sheet tab, but cannot get the sysntax correct to call this from the SUM function. I have stored the sheet name to a cell such as A1, then substituted into :
=SUM("First:'"&A1&"'!B25") ..this does not work, and have tried experimenting with the quotes..no luck.
as well : =SUM("First:'"MID(CELL("Filename"),SEARCH("]",CELL("Filename"),1)+1,32)"'!B25")

Again, no luck. I know this is a simple problem, but I can't seem to find an answer. Can anyone see what is wrong with my syntax ?

Many Thanks for any pointers, Wally