Any question answered...

The length of threading on the old left side compared to the plastic cup is the same, the old one threads all the way in but the plastic doesn't its the plastic BB that needs replacing most likely?

I.e by the looks of it, it isn't the wrong size, its just not threading properly

I think you're right Rogan. Because so little thread is entering the BB shell while your other one can thread in all the way it may be that, as you bought the BB 2nd hand, the cup isn't the one that originally came with the BB.

The metal part may have one type of thread gauge that is compatible with your frames BB shell while the plastic ring has another. (British/ISO, Itallian, French, Swiss etc)

A trip to the LBS would answer to this thread question. They are also likely to have a spare cup or two hangin around. Good luck.