When to prune new portuguese laurel hedge on exposed site

Hi,  I have planted a portuguese laurel hedge & they are quite whippy. I thought I should prune them when I planted them in September, but the nurseryman I bought them off said to leave them until April, then he looked in a book & said no leave it until June. In the new GW magazine, it says to prune on planting & then in July - Aug! HELP! WE live 1000' up in exposed conditions in the Welsh Mountains. I can't decide whether to nip them off now whilst it is still mild or wait until spring. I previouly had a portuguese laurel shrub which had got too big, so I cut it back hard in the autumn & that very cold snowy winter followed & killed it. Any suggestions? Bearing in mind our climate, does it matter if it is left until spring as far as thickening out is concerned?