Using IsNumeric Within A Case Statement (within An Insert)

Hi - Please excuse me if this is really simple, but I'm fairly new to this lark.

My (made up) code is below... I'd be grateful for any pointers.

insert into [tblInvoices]
(full_period,
supplier_no,
account_code,
tran_amount,
function)
select
full_period,
supplier_no,
account_code,
tran_amount
case
when substring(account_code,1,2) = 'FY' then '-'
when isNumeric(account_code) then left(account_code, 2)
when not isNumeric(substring(account_code,1,2)) then left(account_code, 1)
else 'oops'
end as function,
from
tblLoadMSV900_i
end

Is this even close?

I'm using a stored proc to insert the data from tblLoadMSV900_i into tblInvoices and at the same time insert some data into the function field.

In plain english I want make sure that:
If the first 2 chars of account_code are 'FY' then function='-',
If the first char of account_code is numeric then function=left(account_code, 2),
If the the first char is not numeric (and if first two chars are not 'FY' i.e. first char could be 'F') then function=left(account_code, 1)

And there's plenty more where this came from! But if I can crack this with your help then I should have a better idea about the rest of the proc.

Thanks
Sara