[DamnMachine]

No, you're not alone.

Yes, the engineer changed the modem for one of the newer 250s.
He was really helpful, and also commented that the signal levels were exactly where they should be.
Since then it has mostly been well behaved, but twice today the connection has gone again for periods. I was able to watch the modem lights quite carefully. Ready goes out and Sync starts flashing fast. The first time it took four power-ons, a call to ntl and on their advice actually unscrewing and refitting the coax cable. The second time it seemed to come back on its own.

The difficulty with progressing this is that it is intermittent and unpredictable, and naturally doesn't happen when an engineer is around. A neighbour six houses down doesn't have these problems, which seems to me to rule out anything on our spur. And if they've replaced the cable modem and it still happens, that would seem to me to isolate the problem to somewhere between my cable modem and the wiring cabinet on the street.

No idea how to get ntl to investigate further, though.

\Damn