Is that 'new' as in 'recon' and £1000 ish or 'new' as in £2500 ish mostly new parts ? Does it turn over freely without the sparkplugs in on the starter ? wondering if it has been rebuilt with a tolerance somewhere set too tight ? Like the T4 engines that appeared a while back that idled hot for some reason .
Either borrow or make a smoke tester. We use a Bosch one at work to find leaks in the charge air inlet system. They work a treat for finding leaks.