Ian White wrote:

Thanks very much for the replies so far. I really do appreciate how you
lot get your teeth into an unusual problem!

But I misjudged the amount of detail that people might need in order to
give a helpful reply. My apologies for that - I was trying to spare you
all "the full half-hour". Well, here it comes...
snip

Try a cabling company like Nimans etc.
Although I would have thought that simply laying concrete reinforcing
mesh would have been good enough...