I am stuck on how to proceed to meet my clients requirements, all help and suggestions very much welcome!.

ok, just to get the time spans straight, if the invoice is due in November, then they are current, but if it is in October they are 1 month?

Just looking at the boundaries...if the invoice is due Nov 1 and it is Nov 28, they are still current, and due Oct 31 and it is Nov 1 they are last month.

Or is it, if the bill is due after today they are current, and if the bill was due within the last month they are 1 month?

Well let's see what we come up with for both situations:
scenario 1:
report 1
if month({table.dueDate}) >= month(today) and year({table.dueDate}) >= year(today) then "current"

report 2:
local datevar lastmonth:=dateadd("month",-1, today);
local datevar due = dateadd("month",-1,{table.dueDate});

if month(due) >= month(lastmonth) and year(due) >= year(lastmonth) then "1 month"


scenario 2:
report 1
if {table.dueDate} >= today then "current"

report 2
if dateadd("month", -1, today)< {table.dueDate} and {table.dueDate} < today then "1 month"


I might be off on the syntax for dateadd, but these should give you some ideas...hopefully