After Google-ing, Yahoo-ing and even contacting Lagoon in France and TPI in Rhode Island, Nick resorted to pleas on various forums such as Multihulls4us, Cruisers Forum and the Lagoon Catamaran Owners Group on Yahoo.
Graham, on Vinga III (in the Caribbean) answered one of these pleas and very generously made us a photocopy of his 89 page Owner's Manual and sent it to us in Australia.
Nick spent a couple of fun-filled hours scanning in this document today and we have uploaded it to Google documents, so that the other approximately 50 Lagoon 42 TPI owners (or prospective purchasers) can easily have access to it.

Check it out here:
Lagoon 42 TPI Owner's Manual
A huge thank you goes to Graham. We really appreciate your help, and hope to meet you some day!
PS. Having gone to these lengths to obtain this document, we then met Ian (of Skipper's Daughters - another Lagoon 42 TPI located in the UK) last weekend...who, of course, informed us he also had a copy of this 'scarce as hen's teeth' document. Isn't it always the way?!