I write to enquire whether it is a legal requirement that smoke detectors in our church halls complex MUST be replaced now that they are 10 years old. They are serviced regularly and none has been proved to be in any way faulty so far but the fire alarm company we use say that they must all be changed! An expensive business.
I look forward to your early reply.
Thank you,

Smoke detectors should be serviced / changed according to the manufacturers instructions.

Taking Appolo (a very popular brand) as an example, they should indeed be changed every 10 years, this is because that there is a slight possibility they may cause problems.

That said, you only need the detectors changing, not the bases.