Thanks for the replies so far Folks, very useful stuff.
And PB, thanks for confirming what I feared! I may have the simple processing power of an ATCO brain but I had hoped that inputs from elsewhere within the cockpit of an a/c that had suffered a transponder failure would be sufficient to allow it's TCAS to maintain it's functionality. Sadly not!
Keep up the good work
(And if poss, I'd rather it wasn't moved. I originally added it to what may be interpreted as a 'more appropriate forum' and had no replies whatsoever. And there is certainly a serious (and hopefully useful) element to the thread)