The "no flame" neon indicator is flashing (suggests flame failure or
condensate drain blockage).

The condensate drain is not blocked.

If I reset the boiler, and get it to restart, then running a hot water
tap or turning on the central heating starts up the flame and fan.

If there is demand, the flame

* comes on
* cuts out after 10 seconds
* comes back on after about 25 seconds
* cuts out after 10 seconds
* comes back on after about 25 seconds
* cuts out after 10 seconds

.... and then the boiler goes back into fault mode.

The repair manual suggests that the PCB could be the issue; in fact I
replaced that a few years ago, but I can't remember whether it was the
same fault.

Any immediate ideas?

Daniele