find the smallest one downstairs isolate it at the two valves crack open the large nut to one of them prefrebaly the trv one and drain out the water remove the rad valve from the rad or remove the rad from the wall this will leave you with two valves which you can then open and control the flow of water and drain
Probably the easiest way to do it would be to get a self cutting drain cock I found them at bes I haven't found them anywhere else although you might find self cutting washing machine taps at a b&q type store. you would then have to use a washing machine hose. A self cutting drain cock could be left on permanent but I would tend to remove it when drained then cut the damaged section out and solder in a tee with a proper drain cock in it. Have you checked outside? sometimes hidden but can be there.
Don't know if this is any use but the drain tap for my central heating system is under the floorboards in the kitchen near the pump. I usually fix a hose to this tap with a Jubilee clip if I ever need to drain down the system, either fully or partially. If you can locate your pump you may find that you have a similar set-up.