Marshall8p ... about the glow plug or any other unused symbol, I had a look at my spare and training instrument cluster from a Thor engined late V8, there is no glow plug symbol or any other unused one.
I will check that and report in a few days, when I installed the warning LED and buzzer in my GEMS V8.
In the case You don't have a spare symbol it is possible to "scratch" an individual symbol in the cluster, there are spare positions for warning lamps, but blanked out (read non-transparent).
I wanted to have the symbol understandable in the case someone drives my car, without me ... so I've tried a double wave with a triangle pointed downwards in combination with a red LED. Together with a buzzer everyone told me, they understand some kind of low level, which indicates stop driving and switching of the engine. Understandable enough for the purpose.
Funny finding, I don't know where I read that, but the possible error messages in the BECM / cluster display had the programming for low coolant level, but somewhere between specifying the features of the P38 and final production realisation, this feature got lost. As an long term Range Classic driver I liked that feature of a low coolant warning light very much. It saved me at least two engines in almost 20 Years, whereas the temp gauge in my first P38 reacted too late (I guess) when a hose busted with 180 Km/h on the highway. I stopped at once and refilled with water, but afterwards the engine had a bug, which resulted in wrecking the car at the end. Now I have a very nice P38 I want to avoid that kind of failure risk, so an somewhat over engineered low coolant warning feature is worth the effort IMHO ... YMMV ;-)