2004 Peugeot 407 injector leak

There's a tiny little "O" ring on the return,
the bit that goes into the injector,
Either check it or if your eyes are old like mine renew it.
it's really small!!
And I'd renew the spring clip that goes through the injector slot to retain the pipe.
Not a lot of money in that.
If it still leaks after that then there's a fracture in the injector.

hi I`ve replaced the return pipe with a brand new one
it just seems the top is wet

Yeah I got that from your o/p.
So as stated it either has a damaged "O" ring which might have happened when you fitted it,
But as it's leaking from the same injector at the same place if the "O" ring is fine then there's damage in the injector.
That's assuming you replaced THE ENTIRE INJECTOR RETURN RAIL.
As supplied by the dealers the pipe comes as a complete line that serves all four injectors and the return.
The little plastic parts that go into the injectors are very fragile.
that's why you don't replace just the pipes in between the injectors.