units from RO_MAN comes with a self piercing valve / tap
just put the valve over a 15mm pipe and turn the handle bit, this inturn cuts into the 15mm pipe, then the 6mm pipe just pushs onto the valve and the RO unit, of course if you want to move the RO you will then have to fit a 6mm tap unit to stop the water, buy it at the same time from RO_MAN (cost a quid or so ) and tap it to the pipe so it doesnt get lost