Glow Plugs?

Just How difficult is it to change glow plugs on a T4 transporter 1900 cc, 800 special. I have problems in the morning starting the van I was told its the glow plugs !! and that they where a nightmare to change so I took it to a garage and they said it was a bad earth. After sorting that problem out, it was fine for a few weeks. The problem as reappeared. I have plenty of power in the batterie until it drains of course but really struggles when it has been frosty or very damp, any ideas