Delta Anchor won't set.
Thanks so much to everyone for taking the time to share your experience and ideas.
I did spend some time today with 50+m of 10mm chain scope in ~5m deep, sandy bottom. I've got 1.5m from water to roller, so that's technically more than 7:1. Took some revs to get it moving with all that chain, but still dragging along on its left hand side. It's a dud !!
I will be looking for a 'new gen' anchor. I have to buy a new one, so it makes sense to go with the latest and greatest. The final choice is more likely to be a function of what I can get in the local chandleries. Currently in Northern Sardinia, so probably not the widest choice available.