going crazy with leaks at junctions and stoppers.....

[Charnwood]

It looks like a Polygrip Plus fitting, second generation Philmac, which is designed to fit metric MDPE. That pipe does not look like MDPE.

 

[ctp]

The pipe in Spain is black not blue.

 

[TicklyT]

PTFE on the threads won't do anything to seal the fitting. It will just jam the nut before it tightens, and stop the fitting from doing it's job.

Unless the nut runs freely on the thread, it can't compress the rubber sleeve down onto the pipe.

Assemble the fitting without the pipe and look down the end as you tighten the nut. You'll see the hole in the rubber seal getting smaller, and get an idea of how much you will need to nip the nut up for the fitting to seal.

IIRC the grip ring can stop the nut from compressing the sleeve if the pipe is inserted too far into the fitting.

 

[Charnwood]

The pipe in Spain is black not blue.

What does the brown line denote?
On blue water pipe in the UK it denotes pipe suitable for brownfield sites; Protecta-Line, Polyguard, Trigon etc.