We came across a site that is approx 20 years old, they asked us to take a look at the fire alarm whilst onsite and I noticed that the smoke detectors / sounders were wired in twin & earth mains cable. Is this allowed or do they have to upgrade the wiring to FP?

Sounders have always needed to be in fire resistant cabling for a lot, lot longer than 20 years. This is a serious non conformance that affects life safety and must be remedied.

Detection zones to call points and smoke/heat detectors have only required to be fire resistant since 2002 (although most good installers would use FP or Pyro anyway). This is because original conventional fire alarms would go into 'fire' when their cables were short circuited so a fire burning through cabling would still set the alarm off, which would continue to sound as the sounder circuits would be fire resistant.

As long as the current panel is set to operate in 'short circuit = fire' mode (most modern panels default to 'short circuit = fault' mode and would need the setting changing) this is less of an issue unless the panel cannot be set for short circuit = fire

So: Sounders in FR cabling - essential; detection zones in FR cabling - desirable

knowledgeable reply, thanks.