Refer to previous sheet

I've got a workbook that increments in sheets for each new week. I have a cell showing the current week number for that sheet ( lets say cell C1) and any formulas that need to reference data from the previous sheet do in 'indirect' function which basically looks at the current week number ( lets say week 20)in cell C1 and minus's 1 to navigate to the previous week (week 19).

I was wondering if there was a way to reference the previous sheet purely by the order they're in.

So say i have 5 sheets named "1,4,5,8,9" and these represent week numbers so the sheet named '8' in cell C1 would have '8' standanding for the week number but cells that wanted to reference the previous sheet couldn't do the 'indirect' C1-1 as there is no 'week 7'.

The weeks used could vary alot so a formula to reference the directly previous sheet is needed. Hope that makes sense and hope someone can help.


ok, thought best if i show you the actual formula

=IF(ISBLANK(C12),"",IF(ISERROR(VLOOKUP(C12,INDIRECT("'"&($AG$2-1)&"'!$C:$AG"),28,FALSE)),G12,SUM(G12,VLOOKUP(C12,INDIRECT("'"&($AG$2-1)&"'!$C:$AG"),28,FALSE))))

the '$A$G2-1' part is the bit that redirects to the previous sheet based on it's name, what i think i really need is to replace this part with a 'PrevSheet' function but i'm unsure how.

Much appreciated.