In situations like this, small changes can make a very big difference. For one, you absolutely need to cover the viewfinder when taking long exposures, even if the light coming from behind you seems small or non-existent. Remember, it's additive, so even the smallest amount of light over time will have an impact. Secondly, LR noise reduction is nice but is far from state of the art. For times like this with extremely long exposures, you need something a little stronger than LR. I use Nik's Dfine and believe it is probably the best product on the market for long exposure noise.
And I do remember your post and though I'm not an authorized Nikon repair technician, I do not believe you have hot pixels (at least not the kind of hot pixels I think of when I hear the words "hot pixel"). But if you are convinced you do have a hot pixel issue, send it to Nikon asap. But before you go dropping a large chunk of cash on a new camera, I would revisit your long exposure technique and see if these small changes can't make enough of a difference to obviate your need for a new camera body.
I will take the advice above and put it into practice. Will also investigate Niks Dfine and look at purchasing it.
Yes perhaps im wrong in using the term hot pixels as they are not in every shot and not necessarily in the same position in the long exposures though they do have those same characteristics, sparkling different coloured shining pixels.
My my intention to upgrade to the D7100 has been on my mind for a while. I'm not upgrading to try and get away from this issue, I'm upgrading because I want the extra features that the D7100 offers. This originated really because my noise issue was becoming more prevalent in my thoughts and wanted to hear people's experiences of using it for long exposures.
I suppose my issue was/is - is there an issue with my D3200 sensor,or is it behaving normally and the level of noise or whatever it is comes with the territory and has to be put up with or worked on in PP, or do I need to work on my technique to greatly reduce this noise issue I see.
I was was concerned that this level of noise may be something that comes with the nikon cropped sensors and would have no improvement on the D7100.
This place is my only real place for information. Although I am member of a small local camera group, they are all Canon shooters so have no experience with Nikon and they don't seem to have the same issues I am having.
Finally thanks for your feedback. It's greatly appreciated.