I have also been discussing the P50 recently, even today actually. This is my view:
There is a periodic investigation to determine if the extinguisher has been used or not rather than a full inspection, but this is carried out by a user/owner and not a qualified fire extinguisher engineer. There is a difference and the engineer will inspect many other things rather than simply check that the gauge still works and it feels heavy-ish when you pick it up.
The point is that all we hear these days are 'competency, competency, competency' and here we are willing to put an extinguisher in place for 10 years at the hands and competency of a user or owner of a building. It doesn't make sense.
I serviced a school last year and the caretaker had re-tagged an half empty fire extinguisher. He didn't think he had done anything wrong, it still had water in it after all and the gauge was only just in the red! And this wasn't the only one.
Fire alarms suffer from exactly the same, according to the log book, one particular shop that we service do test a different call point every month. But without a key because they hadn't got one!
On both of these occasions our annual or 6 monthly visits highlighted and corrected these problems, so what happens if we leave them un-serviced for 10 whole years.
I believe the standards now state that engineers should carry out a basic service on plastic / non-metal types of extinguishers such as the P50, but the market place and end users don't seem to realise this.