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My glowplugs don't and the engine heater light doesn't come on either,
though the engine starts quite happily with Easystart (don't like using

that too often)

What are the likely causes guys? There's a relay in the circuit called

an 'insulating relay & engine temperature relay, and an engine
temperature switch mounted in the thermostat housing.

I have a limited ability to follow circuits!

reached a certain temperature.

The most likely point of failure in that system, and likely also the
cheapest and easiest to fix, will be the sensor (or "sending unit")
that is measuring the temperature of the block or head.

Is the sender 'normally open' in which case, could I short it out to
see if that is the open part of the circuit?

No, I don't have a manual

Theoretically, the contacts in your sensor will be closed when the
engine head or block is cold. The closed contacts will allow curent to
flow to the indicator lights at your control console and probably to a
relay or control that turns the glow plug system on and off as well.
When the engine block or head heats up, the contacts in the sensor will
expand apart, shutting off both the indicator light and the flow of
energy to the glow plug power system. If the contacts fail to return to
the "cold" position, your glow plugs won't function when needed. You
could experiment with manually shorting the circuit where the wires
connect to the sensor and then seeing if your indicator lights come on
up at the helm. If they do, it's almost certainly a bad sending unit.

Cheers for that - I will try that when next at the boat early next week.

Chuck is trying to be helpful & in good faith so no criticism of him
intended. However be careful, without a wiring diagram you won't really
know how the glow plugs are wired.

Rarely are glow plugs still powered after the engine starts (i.e. use
the 20V dc range of an elcheapo multimeter to ensure NO power is flowing
to the glowplugs after start; none). The block temp sender is only there
to give the system some idea of how long to leave the glowplugs timer on
before the start is made (or even allowed)

Also don't be tempted to hot wire direct to the glow plugs, they usually
have a pretty heavy resistor in the circuit to reduce the voltage
supplied to the series linked glow plugs.

Again with your cheap multimeter you can take the glowplug series strap
off, then on Ohms check the resistance of each glowplug, there should be
some (not open circuit) & the readings should all be similar. Properly
verify the plugs are getting current during the heat period & that they
are all operating, before you go pulling them out on spec, they're
expensive & easily damaged if a bit old & never removed/replaced with
something like a copper based neversieze etc.

K