Have now replaced the wire strop with Teal out of the water. Interestingly there was a bight on the cable where it had over-run and then been trapped by another coil. The wire had to be cut to release it. Not sure if this had stopped a full descent of the keel.
Used Dyneema (Marlow Excel D12 Black Max 78, 7mm with a stainless Steel Eye splice, overall length 4m) from Jimmy Green
Used the full length and there was plenty of room on the winch - and it has held so far!
The eye bolts on the keel seem good for a while yet.
Many thanks to Graham Ebb in particular
Bob