Thanks Jongware and David.
Somehow, I feared that was going to be the outcome…
Anyway, I've got the job and have started making some headway in InDesign with it. I thought I'd share my progress with you and scare the bejesus out of you! I've attached two screen shots.
One shows the document size that I am working to – a staggering 4A0!

The other screen shot shows part of the first section that I have built.

For this, I elected to make object styles for 'Male' and 'Female' entries with placeholder text using various stylesheets and the very handy 'apply next style'. I then grouped these objects with a leader line coming from the top and popped the resulting groups into a library.
Fortunately, the client has requested that all children in one generation run from right to left in a neatly aligned row. This means that I can easily join the entries onto a blood line. Marriages, rather unusually have been stacked vertically so they have a tint behind to help show the union. Where there are re-marriages, the offspring are labelled with a number that corresponds to the relevant spouse – hence the green circles above some of the entries.
I've got a lot more to do as my client has really done his research and this stretches back to the 1400s! Ultimately the family tree will fill the 4A0 sized area, and to give you an idea just how much info there is, the blank 'Male' and 'Female' entries measure 34mm x 20 mm.
Naturally, the client wants it yesterday…. and for not very much money!