Almost every HDi will give that fault message, personally I wouldn't worry about unless you are having starting issues.
All 3 HDi's I owned would start easily without waiting for the glowplugs, even at -15 C or worse.
IIRC the control unit is on the bulkhead somewhere behind the engine, or on the back of the cylinder head.
Guess you could follow the glowplug cables back.
2002 HDi 2.2 Exec Estate, (2008-12) (wonderful)
2003 HDi 2.2 6-speed Exec Estate (2012-19) (also a gem)
2009 Citroen C5 2.0 HDi VTR+ Estate (godawful heap)
2008 BMW E91 330i touring (great fun - murdered by a reversing SUV)
2007 BMW E91 325i touring (slower smoother quieter)
Ahh interesting - in that case I'll leave it be. I did have a minor blip in the winter just passed where it started up a bit rough, but my battery was also very flat and could have been the issue.
I'll clear the code and see if it comes back. I don't really fancy digging behind the engine especially as we're on the cusp of summer
The only real difference you get with working glowplugs is that the engine will idle more evenly immediately after a cold start as the plugs remain energised for a period the length of which depends on how cold it is.
The warning light doesn't indicate they're energised, it's more of a 'wait until I go out before you turn the engine over prompt'.
2002 HDi 2.2 Exec Estate, (2008-12) (wonderful)
2003 HDi 2.2 6-speed Exec Estate (2012-19) (also a gem)
2009 Citroen C5 2.0 HDi VTR+ Estate (godawful heap)
2008 BMW E91 330i touring (great fun - murdered by a reversing SUV)
2007 BMW E91 325i touring (slower smoother quieter)