Draining a system with no drain cock?

Chaps,

I want to add a couple of rads to an existing sealed system.

System is old and I suspect, a DIY job from years ago so it's a right mishmash. The lowest point in the system is the pipework running under the living room floor, which is accesible from the cellar thank god. Highest point is the boiler upstairs.

BUT, how do I go about draining the system down if I do not have a drain cock? Will I have to just crack a joint and get a bit wet? Once drained I can fit a drain point to make life easier.