Q.I have a mature Daphne residing in a 60cm Dia. pot.
My problem is over the last few years it is producing less & less leaves which quickly yellow. The blossoms are in abundance and in evidence the shrub is otherwise healthy. Is this caused by a magnesium or nitrogen deficiency or something else?
Actually, producing fewer and fewer leaves is a sign that your daphne is ailing - it's making flowers in a last ditch attempt to reproduce itself in the face of imminent death.
Not to worry, I think it's a problem that is fixable. So, when was the last time you repotted? A large, flowering plant should probably be repotted evey year or two. The most likely scenario is that the soil is so filled with roots that none of them can get sufficient water; second to that would be that the soil is old, depleted in nitrogen, and/or elevated in salt level. The fix would be to pull the plant out of the pot, prune the roots, and repot in fresh, well-drained potting soil (I like a 50/50 mix of cactus soil and perlite.)
Another thing you can do is check the small twigs and underside of leaves for sucking insects such as scale or aphids, or spider mites; any of these could be attacking leaves but not flowers.
Another possibility is that the plant is simply old. Average life span for daphne is 10 - 12 years, although individuals can live decades longer than that.
Here are some articles that should help you:
https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm
https://www.gardeningknowhow.com/houseplants/hpgen/prune-roots.htm
https://www.gardeningknowhow.com/ornamental/shrubs/winter-daphne/care-for-winter-daphne.htm
https://www.gardeningknowhow.com/houseplants/hpgen/keeping-your-houseplants-healthy.htm
