I've got a radiator with a broken thermostat valve jammed in the off
position - hence the radiator has been off for months. I need to get it
working before winter...

Is it possible on a sealed (Worcester 24i Combi-boiler) system to
isolate the radiator so as to remove the faulty valve and put the new
one on? Or does the whole system have to be drained?

There is another valve at the other end of the radiator. If I turn this
off, and then remove the thermostat valve at the other end, will it
just be the affected radiator that drains, or will there be a flood, or
other problems?

Or what about freezer sprays for isolating pipes?

Or is it a job for a professional?

Thanks in advance - any tips or advice welcome!

William