Right Starry. It's really stuck down then? Even if you wrench it free from its seat, it'll probably never shut off again.
You'll need to go out to the pavement and turn off the Water Company's valve. Might be an old-fashioned one like yours, or a newer quarter turn type (probably plastic).
If you're not confident enough to remove the whole stopcock complete with backnuts and olives .......... then leave them on the pipe......... wrap a few turns of PTFE tape around the olives, and just fit the new valve body part only.
The threads should be standard 1/2" BSP, just like the new ones.