>>(on my meter it shows as a high negative number that's dropping).

DieselAddict, that is an excellent suggestion. Thank you for describing it.

I've been thinking on and off for ages about how you might cheaply and simply be sure that a glow plug is actually glowing properly and getting hot without having to go to the bother (and risk on an OM606!) of taking it out.

What you see on the meter is, of course, the glow plug behaving as a thermocouple, and producing a small voltage in reaction to the heat (as per the milli-volt method described by Barry1234) - hence your meter reads a changing resistance as it cools - absolutely brilliant!
Top banana!