Alright, I just read another post by you in the Visa section... congratulations.
Here's what happened for me as far as health visitors:
My midwife explained that basically the midwife is in charge of taking care of me from pregnancy to about 10 days after the birth. There is a bit of an overlap (in most cases) after the birth of your child - you may see the midwife a few more times or you may not, it depends on your personal situation. You will be visited by a health visitor shortly after the birth, they will contact you (you don't have to do anything!) and will come by your house. She will chat with you, see how you are doing, answer questions, and be helpful. She will give you handouts probably. She will weigh your baby as well. Mine gave me a little written test to watch for Post Natal Depression. If you need to see her more (you'll talk about this, again it depends on the situation) then she'll set up another home visit with you, otherwise you can contact her (or other health visitors) from their main "headquarters", usually at a local GP's office or hospital. You can reach them by phone, and a lot of them have walk-in clinics where you can go if you have questions or want to get your baby weighed. If you don't like your health visitor (as with your widwife) you can always ask to get another one though it seems to depend on your area as to whether or not that is likely.