Let's take this as a scenario - Your L200 comes back repaired, with the 2nd hand shocker fitted, and you have an accident due to the shocker failing. They in turn then say they are not paying out because of inferior parts on the car. That leaves you with the bill. I have just had my tyres replaced on my van and was asked if I wanted normal car tyres for the front rather than the van tyres I was replacing, perfectly legal. The mechanic said that if I chose normal tyres, that my insurance could be invalid in the case of an accident. I couldn't see anywhere on my policy where it stated that. So it appears insurance companies can get away with putting on worn/2nd hand/inferior parts and get away with it, but if the boots on the other foot....We always end up paying no matter what.
I would say go for it Doug and don't give up until you get the right outcome! You've probably paid thousands out over the years and received very little back, apart from the odd keyring or Parker pen!!!!