One thing i noticed on my Disco 1 and now my 90 is that the Glow Plug Indicator lamp on the dash stays on for x seconds but if you listen you can hear the relay under the bonnet 'click' off after a further possibly 10 seconds. I assume this is a standard thing? Is the indicator a set time but the relay is dependant on the outside temperature?

Yes, the dashboard light doesn't necessarily correspond to when the plugs are actually switched on. Or at least it's that way on my TD5. If there's any influence from the temperature, I would expect it to be engine temperature, because I don't think the ECU has an external temperature feed to base it on.