No. I need one that works on my line, and as this one did so faultlessly up until 14h20 on 31 March, and all other routers (including the one that's connected to it right now) have done so without too much trouble over the last 20-odd years...

That's not the point - trying it on another line is to eliminate or confirm that it's something local that is triggering the issue. As this is an unusual problem, it's very difficult to diagnose so elimiation is a key tool. An incompatibility isn't a fault - one thing working where another doesn't isn't proof of a fault either. It might be that some other device at your location is out of tolerance, but if your attitude is that "the router should just work I don't have to help eliminate causes or narrow down the environement" then it's impossible for them to help you if they were unable to reproduce the problem.

admin wrote:
An incompatibility isn't a fault

Yes, it is. The VDSL interface standards (there are more than one) are well-defined. Irrespective of what's going on with the LAN, the packet losses and latency between the router and the DSLAM and beyond are abnormal. Remember, the Firebrick doesn't have any interest in, or contact with, the LAN - so there should be relatively few lost packets (there will always be some, that's the whole point of using Internet Protocol) and "normal" latency.

Certainly, latency of over 1s is a serious fault, and the "swap out" with another router resolves it.

Quod erat demonstrandum.