I have the Electronic parking brake warning light illuminated along with the cruise control light flashing and engine management light on. Went to garage and they say it is showing up as 'Glow Relay Circuit Low'. I don't have any parking brake problem, the car starts and drives fine at the moment, although cruise control doesn't work. Should I consider changing the glow plugs? Anyone with a suggestion? Any idea how long it would it take me to do the plugs? Going on a 500 mile trip week on Saturday! 

I had exactly the same issue.
The parking brake is unrelated. But yes, you have to replace the glow plugs, that is most likely the rootcause.
My local mechanic explained to me that, as they age (when over 100,000km), their internal resistance reduces, hence they would gradually be drawing higher current, which would trip the glow-plug-fuse.
Got all 4 plugs replaced, new fuse, and no problems 20,000km since