This morning I rang F&H and explained the weight anomaly; they were unaware and agreed to pursue it with the factory. Coincidentally, an hour later another beefarmer rang me with the news that he'd noticed that the old F&H is shorter than the new one, and that he'd also spoken to F&H. A warehouse check confirmed the difference, which in a photo looks to be 3-4mm, enough to alter the volume.
Exactly, and F&H agreed that moving the goalposts is not on; the beefarmer above has labels printed 3000 a time so must either give away 10g per jar to maintain the fill height, sell jars looking short or dump his labels.
10g x 3000 is a bucketful of honey which to me is worth about £400, to John more as his prices are higher, so the long and short of it is that jar dimensions, quality and origin matters.