Has anyone had success growing Thuja plicata atrovirens in the UK as a hedging conifer? How fast did they grow, and did they form a decent bushy hedge?

I planted some a few years ago as I thought they would be a good alternative to leylandii, but have been very disappointed with them. They have only grown very slowly about 6-12" a year and not very bushy, they have a good framework of branches but very sparse foliage, so I can easly see through them, will they thicken up eventually? or is it time to replace them.