TD the letter does make a difference. DD2 had one when she was so ill just after she come home from overseas and she only had to wait a short time before being seen.

I agree with Eluned, you don't always have the vomiting and fever so take care and listen to your body.
If Mar... is your local ring before hand and ask how busy they are and tell them why you are asking. We have found that sometimes this helps if they tell you to come in