Hi Frascati, portuguese laurel or prunus lusitanica, can grow up to 70ft tall x 70ft wide, so if you're going to let them mature you need to plant them at this distance, they will form a boundary but not a hedge, and keep in mind that they're not as hardy as prunus laurocerasus, common laurel, Derek.

Thanks Derek.
We are currently growing a beech hedge 3 metres in front of them (it's been growing 5 years) which we intend trimming & keeping at approx 6ft high, in addition we want something for extra height behind them. All literature i have come across tells you what distance to plant them for a hedge but not for mature trees.

As a hedge think of them in about five years' time, somewhere between 6 and 8ft tall. Allow them a proportionate distance between for good growth, roughly three to four feet. However, to allow them to mature, and they are magnificent in maturity, might mean you start as above for screening, then remove every other one after ten years. As F says, mature is pretty tall if you're a portugal laurel. I'd go for a wide spacing to allow them to mature proportionately, as I hate cutting down healthy trees if I can avoid it