On Wed, 9 Oct 2019 21:50:12 +0100, Mike
wrote:

snip

[1] If you peer into the boiler though the spy hole and watch
carefully, every 3 or 4 sparks it looks like it is surrounded by a
tiny flame but as if it's blown out before it can build up?

Mine has exhibited these symptoms. Here's what I do to mine.
Isolate from electricity.
Pull off the thermostat knob. Remove a 'rubber' bung to reveal a screw
which holds on the control gear front cover. Remove the screw (mine is
captive)and the cover.

I have been in there a while back when fitting a replacement
controller PCB so that sounds familiar. ;-)

There is a clear plastic tube which passes through the steel backplate
close to the spark generator black box. It goes into a pressure switch
(a disk about 75mm dia).

Sounds familiar.

Where it passes through the backplate, the
plastic tube is jointed by what looks like a bit of brass tubing. But
this is a restrictor, with a very tiny hole in it. Pull the clear
plastic tubes off it and blow it out, or carefully probe with a fine
needle.

I think I may have a 'pricker' somewhere but she might also have a
good range of needles if I can't find it.

Job done, I do it routinely about every 5 years.

Fingers crossed then. ;-)

You can test if the boiler starts by turning on the electricity, there
will be live terminals exposed, and sucking on the tube that goes to the
pressure switch.

Understood.

Re-assemble.

Reverse of ...

Hope this fixes it.

Me too.

So, is it simply the fact that something doesn't think the fan is
running and pulling a sufficient vacuum on that side of the boiler to
turn on the main gas valve?

Cheers, T i m