Current British Standard codes of practice advises that an extinguisher must be refilled with a manufactures approved or verified refill (i.e. not a generic, unapproved, cheap refill which arguably was allowed in the past). Combine this with the engineers’ time and environmental disposal of the contents, if it’s an extended service, and you have a pretty expensive procedure. Compare this to an engineer just exchanging your old extinguisher for (in most cases) a brand new, guaranteed extinguisher and environmentally disposing of the complete old extinguisher which is much quicker and convenient for your service company.