Good advice - I did mine when the engine was warm as well, which was another thing that was recommended. I've heard some people recommend using a torque wrench set on reverse to around 15-20Nm - if memory serves, they shouldn't be torqued anything more than 15Nm, so if they're not coming out at 20Nm you may be on the verge of snapping them. Beru glow plugs have a failure rating of 35Nm though, but you don't want to risk it - better to spray some more penetrant, leave it for a few days and get the engine hot again before making another attempt, rather than have to deal with a snapped plug in the block.
It sounds silly, but make sure you've got the right tool to take them out too. If memory serves, they're 10mm. I was going to buy an actual glow plug tool, but I had a deep-section impact socket that was long enough to get to the block past the intercooler boost pipe.
I undid the brackets for the boost pipe too - access can be a little difficult for #3 and #4 so I took the bracket off and it was enough to lever it out of the way.
Oh - and when I took the harness off, I did have bits of plastic stuck around two of the plugs, which might have been part of the harness. I couldn't get a socket on without removing these, so used a pick to get it off. A long screwdriver will be fine if you haven't got any of these. I haven't had any error codes since, so even if the plastic was part of the harness, it'll still connect up again OK.
