Okay - update! I just fixed it (I think). In the interests of expediency I just decided to take out the relay and open it and visually inspect the guts. I mean, it's just a relay, how hard could it be?

I should mention that prior to taking the relay out I just checked the operation of the glow - I wanted to see if the light was reliably not coming on, or if the system was just 'flaky'. It turns out that the two previous occasions I tried to start without the light seem to have been just fluke happenings. I tried glowing again 5 times in relatively quick succession, and got the light each time.

This led to my thinking "maybe the relay is just a bit dirty from arcing". The relay looked to be the original one (so about 24 years old) and on opening it, I was right. The power contacts were dirty (but not excessively so) but the mfg. date on the relay says it was made in 07/83.

A bit of fine sandpaper on the contacts, and a quick reinstallation, and I got 5 good glows in a row (and successful easy starts like I expect). So so far, I've either fixed it, or not made it any worse. I may have just saved myself the cost of a new relay!

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1984 300D Turbo, 326000 Km, Thistle Green Metallic