Easiest way would be to buy a flexi tap connector hose with integral iso valve and attach it onto the copper below the speedfit connector, discard everything else there.
That looks like it has a internal screw for which you will need an Allen key to adjust.
You will need to unscrew the whole thing to repair it, using a new O ring, or maybe PTFE tape,
I changed a a rad for a heated towel rail for someone today, had to alter pipework slightly to make it neat.
Used the bungs as shown above, lost about a litre of water before it sealed, job done.
If you've turned the valve clockwise, you will have turned it off.
Some of those valves can be repaired when they leak, if not, you'll have to replace it/them.
Have you tried opening it fully? Sometimes they stop leaking when fully open.