Thanks for the pdf information, illysmanx. The more I read up on vit D, the more I realised I probably have a serious deficiency! I went out and got some D3 capsules, and I'm seeing a nurse for a general check up, including hopefully a vit D test, next week.
I feel quite annoyed it has taken me so long to find out about this- the information should be out there. It's amazing the amount of junk they'll teach you at school, but won't teach you about things like this which are crucial to your health and wellbeing!