Is honours in maths necessary for admission in the MSC (financial mathematics) from MSU Baroda?

Yes, in fact Bachelors is necessary for any Masters course. So in order to get admission in MSc Financial Mathematics, you will have to complete Bachelors in Financial Mathematics first. 

Now coming to your question, Honors in Margs is necessary for admission in MSc Financial Mathematics, I don't think so because I have many friends who are pursuing MSc in Financial Mathematics despites graduating from universities that don’t even offer Honors in Maths. I think all you need is a graduation in Maths.

For detailed information look up on the official website of MSU or contact an alumni.