I have an empty 2011 NU-SWIFT 6 litre plus additive extinguisher.  It was recharged last year but used since. I've replaced it with a foam one and wanted to know if it's of any value to someone.  Or it goes down the tip. 

Take it down the tip.

They are so rare (due to price) that whilst current they are technically collectible for those in the trade that are collectors of historical stuff like me, but realistically its only fit for the tip.

They can cost £450 up to each and no one other than Nu Swift (or another member of the London Securities group) can correctly service them or recharge them as they use proprietary parts not available to the trade. For example, most other makes of cartridge operated extinguisher (rare to see these days too) use a pretty standard set of CO2 cartridges, I can use the same 55g PVC CO2 cartridge across about half a dozen makes of extinguisher. Nu Swift instead use a cartridge filled with N2 and a non standard thread.

I note TYCO recharged it, but must have used a cartridge from another scrap unit and a non verified additive solution. The cost to get it filled with new parts and components is such it would be cheaper to give you a new unit.