Thanks for all the comments, although I'm not sure I'm much further
forward.
A few points relating to the comments. I started by cleaning out the
insides getting rid of the incinerated bugs etc. There no sign of an
electronic flame failure device or any sensor listed for the Solo2. I
had already tested the thermistor by immersion in hot water and got a
range of values that seemed reasonable (but I don't have any data on
the correct values). I orginally suspected the thermistor from the
fault finding chart. I must say I am still a bit suspicious of this
part of the circuit.
As far I can tell there is no obvious relay chatter - nothing audible
anyway. The ignition fires fine when called on to do so - the problem
is that it isn't being called on (i.e. the pilot warning lamp is off)
Now the weather has turned colder there is some evidence of the the
boiler going out/switching off soon after firing up i.e. very short
cycling times. From the outset it has seemed that the problem was
usually seen when the boiler had had to work hard - on stating in the
morning, etc.

Pete