Not much servicing required. Weight them to check for leaks. Is there a small pressure gauge on the extinghuisher, with a red and green sector? Just check the needle is within the green sector. Damage/corrosion is unlikely. Check the mounting for secuity, if used. Instead of paying to get them serviced, probably as cheap to replace them every few years (5yrs)

This was the advice from HSE that confused me, IF a transport unit is not carrying dangerous goods in accordance with ADR 1.1.3.6, it has to be equipped with at least one portable fire extinguisher for the inflammability classes* A, B and C, with a minimum capacity of 2kg dry powder (or an equivalent capacity for any other suitable extinguishing agent) suitable for fighting a fire in the engine or cab of the transport unit [ADR 8.1.4.1(a)] Whilst we have no particular dangerous goods we have amounts of grease,oil etc. associated with servicing of work equipment. The replace every 5 years sounds sensible though.