WHULF said:

I got a friend of mine who knows more about electronics to explain where the diode would need to be, slowly and with pictures, so I’m happy that I’ve got that nailed down, I’ll throw a diode in anyway, just to be extra safe.

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The diode goes across the relay coil but in reverse bias. A diode will only allow voltage from the anode to the cathode, or towards the band end of the component.
When used to protect against back EFM, the band would be placed on the positive wire of the relay coil. In normal operation it would be effectively out of circuit, however when power to the relay coil is removed, the coil will generate a high voltage in the reverse direction (call back EMF), at which point the diode conducts and clamps the voltage down to 0.6 Volts, eliminating any possibility of damage to the shutter drive circuit.

I hope this explains its function.

I'd draw a diagram, but the forum isn't letting us mortals post pictures at the moment.

That’s ok thank you my mate drew me several diagrams until it finally went into my head. I’ve just been setting out the relays and fuse block in the container I’m going to replace with something sturdier in preparation for wiring it all together… once I’ve got the spade connectors that I stupidly forgot to buy the other day.