I was curious to see how the valve worked so I took my old one apart. Turns out the one I bought as a replacement was exactly the same as the one I took off (Blue Print).
Interestingly my symptoms weren't consistent with a totally broken HCV. Generally speaking the heater control behaved itself and did what I wanted, albeit slowly. However if I gave the car some right foot treatment I'd always get an increase in air temperature corresponding with the harder work for the engine.
Then I found out why. There was a piece of old plastic pipe lodged in the valve body preventing the plunger from fully opening (or closing, I'm not sure which). Presumably this is the remains of the original Ford HCV body that collapsed when previous mechanic was changing it. That's why it's important to get rid of all the bits of broken plastic! I would recommend a flush if you're not sure you have recovered all the broken bits of pipe.
Anyway the silver bit is just a solenoid controlling the movement of the plunger, which opens/closes depending on what you've selected on the heater control.
Having opened up this Blue Print one I'm satisfied that the construction is reasonably robust, in fact I'd say the valve body/spigots are actually tougher than the Ford original - there was no sense that it was going to collapse as I removed the pipes.
Anyway even if I'm wrong I now have a spare HCV valve (now working fine since the errant bit of plastic has been removed)!