Hello English Dave, while you are waiting for a response from mrsD, here is a link to an article that covers how much vitamin D you may need and how to improve its absorbability:

http://www.wikihow.com/Best-Absorb-V...-D-Supplements

The information is fairly basic and there are some annoying pictorials, but there are also some good ideas contained in the article. Here is once excerpt:

“People who take vitamin D supplements run the risk of vitamin D toxicity which can cause an unsafe level of calcium in the blood. It is not recommended that you take more than 4,000 units of vitamin D per day for an extended period of time, and your required dosage may be much less.”