newbie - 15mm plastick pipe Softener inlet (Mains) leak - Help!

I discovered a leak from a small section of plastic pipe. Its part of a horrible job some "professional" did about 12-15 years ago when the kitchen was extended.

Unfortunately, it seems the builder / plumber (also) didn't fit a (new) stopcock inside the kitchen... I'm going to dash ou now to get a ket got the stopcock on the driveway right now in case leak gets worse!
The leak is on the small section ogrey f plastic pipe just abobe the levaer ball valve on the left (which is above the white clip).

I need to:
1. Stop the leak, Options i guess are
a. turn off the mains using the existiong ball valve, and taping the pipe as a temprary measure, or
b. use some repair kit / gadget to effectively sleeve the leaky plastic pipe, o d
c. attempting to replace the section of 15mm pipe - presumably get some 15mm plastic pipe, and cut it to size and replace the section using existing compression components

2. Support the pipework so chances of another leak developing are reduced. I guess I need to but some wood and fit to (stud) wall or something else form like shelf / floor tile, and fix some 15mm pipe clips.

Any guidance welcome. I am an engineer (electronics / software) by background, but currently unwell (the water softer/ leak are not very accesible), but never really done much DIY / plumbing.

Not the best job on earth!
Just get a couple of 15mm compression cones short end of 15mm copper and replace damaged section.
Maybe a couple of wraps of ptfe on cones before assembly.

No need to go outside.Just turn the full lever on the left of picture and do as stated above with a piece of copper and two new olives.

Have you tried to tighten the nut? Those existing pipe clips look too big, as in 22mm. Maybe change to 25mm if they are?