Following Carim's suggestion I rewrote my array formulas to include */
+ in place of and/or and now they are working fine. Thanks too for
that sumproduct function and that trick of highlighting part of your
formula and evaluating it with F9 - it's very useful.
It seems that the mistake I made was about thinking that {OR{t,f,t,f},
{t,t,f,f}} evaluates to an array of {t,t,t,f} and not to the simple
{t}

Tom