Amazingly the new Livebox was ready to be picked up from le Vigan today and after a couple of hours, success.
The new box is up and running, with renamed id and password (rather than the usual ridiculous string wretched visitors have to type in), the Netgear wifi extender has been reconfigured to the new box and all devices connected.
Most satisfying of all, I’m getting an internet speed of about 7-8 Mbps rather than the <1. Not exactly an intercity express, but certainly a solid reliable goods train. To check it worked I watched the second half of the Nadal-Kyrgios match without a single hiccup. No more horrible buffering symbols.
This all goes to show that once again an Orange customer was right and the technical service wrong: the problem lay with the Livebox.
Yes, I know I am a sad case of a techy nerd, but it is such a relief to have the internet working reasonably after months of problems. It has made me realise how much I depended on it.