Peterspots. - If that is a UK measurement , from a UK lab, then it will be in nanomoles per litre (nmol/L ). The other units of ng/ml is American and would not be reported in those units from a UK lab. You are right 30nmol/L is equal to 12ng/ml.
A reading of 30 nmol/L is very low. Especially since we are in Mid June , and you would have been expected to have raised your level from sunlight, by at least some, from the end of winter. You can actually start making vitamin D in your skin from mid April. So in the depths of winter , you were very possibly below that reading of 30.
Your treatment of 3000 IU should ( if your response is an average response) , raise your level to about 100 + , after about 3 months of taking it.
Some doctors will give a loading dose to "kick start" the recovery of your level. i.e. 20,000 every couple of days for a few weeks. Those patients will reach a higher level more quickly. Your doctor has chosen not to do that with you. After about 3 months however you should have reached a pretty acceptable level. Many Vitamin D researchers believe that 100 - 150 is the blood level to aim at. ( This is not the consensus medical profession view, but it is the view of many many Vitamin D researchers and others. )
Those researchers would look at your level of 30 and call that very low, a deficient state, it could be worse admittedly , but it should be a heck of a lot better.
There is a distinct possibility that low vitamin D is causing some or all of your problems. It makes a great deal of sense for you to get your D level up. I would also suggest you get a bit of sunshine on your skin. Not only will it help with vitamin D levels , it will produce other beneficial effects such as anti-depressive endorphins, etc. Do not burn, that is not helpful. A few minutes , (without sunscreen), when the sunlight contains UVB , i.e. between 10 and 3 pm. in the summer will help you build up your D levels as well.
Hope this helps. Good Luck.
Have a look at some of my other postings, on other threads, I have tried to give a fuller picture of Vit D and its effect on the human body. etc etc.
BTBH