then any knackered ones will be refurbished and put back in.
I would just replace the faulty injectors. There are very few parts available from Bosch, so refurbished injectors have a mix of less worn parts from different injectors fitted to make them functional.
These "refurbished" injectors have a very short life compared to new injectors, and the spec characteristics will be incorrect from the numbers on the top. This is less than ideal, especially as a refurbished injector costs ½ the cost of a brand new injector.