Update: I have now found the relay and control box! At least, in a photo. It is very well hidden!!
It is beneath the battery, (as
@varesecrazy suggested), but not connected direct to it, and you can only reach it if you remove the battery, the complete battery tray and associated ECUs attached to that, push other wiring connectors out of the way, and then you can, just about see it!
I was helped by this diagram from
ePER, where item 1 is the relay/controller. Take note of the metal bracket, item 3... (which I thought I'd seen before somewhere)
That bracket can, just about (!) be seen in my photo from an older post about sorting a stiff gear change, pasted here. It's as if that bracket was designed for something else, and so most of it isn't used!
The wiring to the relay/control unit comes up from underneath: there are two plugs (apparently): one connected to the glow pugs and the other to the main engine ECU via the wiring loom to the big plug top left of the photo. According to the circuit diagram in Haynes AutoFix, there are inputs to the glow plug control unit from several sensors (air temperature via the MAF sensor, oxygen sensors above and below the diesel catalyst, the 'variable nozzle turbo motor', and the oil pressure control valve) and it appears to use these to decide if the plugs need to be heated, and for how long – the days of a simple timer for glow plugs seem long gone! I haven't actually been to look at it in the car (it's raining again!), but suspect it might be easier to reach from underneath after taking the 4x4 undertray off? All I want to do is check continuity from the plug on the relay to the glow plugs themselves...
The photo above is the view AFTER taking out the whole battery tray and main ECUs. Here (below) is what it looks like while they are still in situ -- which explains why I couldn't see the relay unit! (The 'unrelated plug' from the photo above can be seen at the bottom of this second photo, where it's been unclipped from a bracket on the side of the battery tray)
