has anyone had a go at setting up a formula to check that a reference number
is valid via a modulus 11 check digit? i can't find that there is a function
specifically for it.

Michelle

Michelle,
I just saw the algorithm in the internet
(http://docs.hp.com/en/32209-90024/apds02.html)

the following formula calculates the check digit of a number in A1.

=11-MOD(SUMPRODUCT(VALUE(MID(A1,ROW(INDIRECT("1:"&LEN( A1))),1)),
MOD(ROW(INDIRECT("1:"&LEN(A1)))-1,6)+2),11)

To append the check digit to the number

=value(A1&_formula_)

HTH
Kostis Vezerides