Prof., the owner only said he hadn't been told. Maybe it was easier to tell you this than ' I was told about this years ago, but decided to ignore it'!
Thanks Wiz and you make a fair point over the purpose of the dual circuits. In this case the owner has a full set of bills invoices and reports dating back to when the old Firedex 1000 was installed so I can confirm he has never been told in writing. Actually thinking about it, the installation probably predates the 88 standard so that may be a part of the explanation. Numerous original detectors remain as well. 
Prof., it is true that an older standard didn't require more than one sounder circuit, but I can never remember when the new recommendation came into play. I'm sure Mr CT will know the answer.
However, if the system is that old, I agree that he should have been made aware of the variations to current standards by his service providors. Maybe the next BS revision should provide a recommendation that a full current BS compliance inspection should be carried out every 3 or 5 years or so?
