I'm pleased to find you have a decent contractor! Too many 'rag & taggers' and even usually more decent firms are oblivious to this new(ish) requirement.
Damaged and degraded plastic valves have and will undergo critical failure and engineers have been severely injured. I see too many awful ones in service waiting to pop.
This issue is what finally saw the demise of the well used Fireward plastic bodied dry powder extinguishers that were sold between the 60's and mid 80's by almost everyone (but most commonly Betterware, Pyrene/Chubb and Angus).
At extended service they should be replaced or before if obviously damaged. The TG Series 2000 and the Ceo Deux valve (used over the years by Allied, Angus, FPS Bristol (Firemark),Gloria,Firesnow,etc) can still be obtained to do this, the other common ones such as the pre 92 Chubb, Series 2000 1/2 kilo powder and more obscure stuff isn't.
Most people who follow the guidelines simply condemn (after all most places service exchange at 5 years anyway now, although it isn't really an exchange as your goes in the scrap & you get a new Chinese model in it's place) as it's too much hassle and not enough mark up to stock and replace heads.
I don't either, but offer a suitable discount against replacement, although bear in mind unless it's a Gloria cartridge model your original extinguisher will be over 10 years old....
I've seen a Chubb engineer replace a S2000 cartridge water headcap at Extended Service so it does happen (although unless on a local authority contract it will have cost more than new equipment from other places!)
