Part number or supplier

Got mine off eBay, didn't go for the cheapest but looked for a recognisable make

Did the same as Chris......but found out my local factor could do same ones at same price after the event. Cant see there can be too much variation in this type of thing.
As Pete said in your other post check voltage at supply to plugs as well.

I use NGK plugs in mine.

Euro car parts are doing dense glow plugs for £15 a plug for a 2.8. http://www.eurocarparts.com/glow-plugs. ECP part number: 438720080

I bought what I thought was a set of plugs from ebay only to receive one in the post.
The photo was very misleading showing four and I assumed that the 1 quantity listed meant a set.
When I emailed the company denied it was misleading and offered to take it back but it wasn't worth it because of the postage.
They haven't changed the photo.

Rob, on our 2.8s the electric feed to the plugs is a bar with the feed wire connection at the bulk head end rather than each plug having an individual feed wire (the joys of no ECU crap :thumbup:

), thats where I would get the multimeter connected and make sure your getting a good feed first.
If you do the plugs there's a very small nut on the top of each plug that holds the leccy feed bar - think 7.5mm thereabouts. Put a bit of bluetac in the end of the socket to hold this small nut should you undo this small nut fully without realising when it's loose. If memory serves me correct, I think you can undo the small nut slightly and the bar will slide out. You will find it easier to remove the plugs with a long reach socket and UJ connectors.
Whilst your at it, personally, I would have the rocker cover off and do the tappets. Buy a new rocker cover gasket set because you not only get the rocker covers rubber gasket, you get the new grommets where the bolts go through. I paid £3 odd + a couple of years back from Jayar and it's been fine.
As the guys say, get a branded set of glow plugs.