Tier 5 Charity Worker Visa extension

Good day,

I'm currently on a Tier 5 Charity workers visa and would like to extend it, my current one is valid for 7 months and I'd like to take it up to 12 months.

1. I would like to know if I need to get a new CoS number before I apply for the extension?

2. I didn't have to pay a healthcare surcharge with my initial application but I will have to now because of the extension. Which dates will I have to put in, from the start when I arrived or from the end of this visa till when I want to stay?

Any help will be greatly appreciated. I can't find these answers online.

Thank you
Simone