I have been taking 300mcg of Biotin daily for about three months now. It is difficult to quantify the results of taking it, if any, so far. Experts suggest taking it for at least nine months to judge its efficacy.
Anecdotally, I believe it has helped with bladder control and the fatigue is less pernicious. I have some mobility but I cannot swear to any improvements there.
Take a look at the Facebook page that has some useful files about dosage, how to measure the dose and how to obtain it in concentrated form.
You will need to buy it in concentrated powder form and use a measuring device. I am sourcing mine from PureBulk in the USA. Postage is cheap but be aware that you might incur either a customs fee or VAT when it arrives. So far I have made two orders, one attracted the attention of the Border Agency but the other did not.
On the Vitamin D front, were you recommended a particular dose? I take 5000iu daily but others take up to 10,000iu. You should have been advised not to take Calcium supplements with it in case calcification sets in. After a few months on a high dose, ask your GP for a blood test to check your Vitamin D level and your Calcium level.