Please can you point me to the relevant legislation and guidance for Freeholders (some of whom are also landlords) of a small block of 12 separate flats. About 50% of the flats are let as rental properties.
Could you also let me know if the Freeholders should be commissioning a regular fire risk assessment (if yes how often) and does this legally need to be undertaken by external professionals?

The Regulatory Reform (Fire Safety) Order 2005 is the appropriate legislation for the common areas of blocks of flats and the guidance is Sleeping risks or Fire safety in purpose-built flats..

The Responsible Person (RP) is responsible for conducting an Fire Risk Assessment (FRA) and it must be reviewed regularly or if there are changes that may affect the FRA. There is no guidance what regularly means but it is generally accepted about every twelve months.The RP is the person or persons that have control of the common areas (ie. who is responsible for the maintenance) it could be the owner, freeholder or an agent who looks after the building.

The RP can conduct the FRA, if competent or have an externally competent person to assist.

Check out http://www.firesafe.org.uk/regulatory-reform-fire-safety-order-2005/ for more information.