1980's - that explains it - if it was certified as acceptable when first certified in 196- or 197- which it would have been, then unless there was some massive change to the premises the FRS could only fire off goodwill letters and it's no surprise the case failed.
Plus even with a change to the premises it may not have been enough to rip the whole lot out - the most common scenario would be the changed or new bit got a small 24v DC conventional system bunged in 9if AFD was required) and it would be interfaced back to the 240v system for the rest of the site that would remain unchanged - seen it a lot in offices through the 90's where the occupied floors had their own seperate new 24V conventional systems added by various tenants all linked back to the landlord's central system that would be the same old Gent or AFA 240V system that had been in for the last 30 years with the old metal 'drop flag' C&I panel
