mumintraining - there is a product that is availabe from your chemist called 'Gatrolyte' comes in different flavours.

I had a really bad case of Gastro about week 26 of this pregnancy and was finding it hard to keep anything down.
Got DH to go to the chemist and get me some of that and it helped heaps!
It is actually specifically designed to replace lost electrolytes - has potassium, magnesium etc

Maybe try giving that a go? I was getting really bad headaches as well and after forcing that down me it made me feel sooooo much better!

Hang in there hun, hoping it passes for you very soon!

P.S My girlfriend has 13 days to go (2nd pregnancy) and is still vomitting. I guess some women are more prone to it than others sadly!