Hey Greg, it really depends on the type of extinguisher. If its a Lifeline or SPA then you can send it to www.merlinmotorsport.com who are based at Castle Combe. Not sure about demon tweeks what kind of extinguishers they service.
Also you can send it back to the manufacture and they should service it no bother.
May be more local places afoot, seem to recall it being mentioned but cant remember where