High Andy,
Sounds like a lazy tappet to me, not uncommon on CVH especially if it's had infrequent oil changes or lots of short journeys.
DON'T try to fix it with flushing oil or they'll all get clogged. Change oil and filter for good quality ones and get it nice and warm then go for a good long drive. If it don't quiet down they you'll need a new set
BV.