Leaking 15mm to 10mm internal reducer

[Cakey101]

After some decorating and refitting the radiator, I have a tiny leak from a 10mm copper pipe with a reducer that has no olive.
What would anyone recommend?
The solder ring reducer as posted in the links above?
Never done that before.
Cheers.

 

[RandomGrinch]

Sorry, just to understand, do you mean it has one of these reducers: https://www.screwfix.com/p/flomasta-compression-reducing-internal-coupler-15-x-10mm/70435
connecting a 10mm pipe to your radiator valves?
Or have I misunderstood?
Edit:
I have misundeestood, as you mention a solder ring reducer!
Can you post a picture?

 

[Madrab]

Hate those snap reducers, if they're ever loosened they always end up leaking.

PTFE tape is the only real option here, wrapped around the top and bottom sections that fit into the valve and nut respectively or ideally cut out the piece of pipe and replace using a soldered reducer if possible.

 

[RandomGrinch]

When I've used the reducers in new joints, I've smothered them in Fernox LS-X first, as belt and braces. I don't know if this will help on an existing leak though?