OK, I've finally found my head diagram.gif from my manual.
On this diagram, if you remove the COV valve and replace it with a plug, then you'd have to have a plug machined to fit the back end of the expansion chamber precisely. If you wanted to fit a glow plug into this replacement plug, it would have to be an awfully long plug. That said, I'm not familiar with glow plugs, maybe they are that long. Also if that "inner sleeve" (part #148, see walktall's great new plate #4 page 37 Cylinder Head diagram, found in Lister CS Plates under Original Lister CS Engines) comes out easily and is replaced by the plug, then it's a different kettle of fish.
So if part #148 comes out and is replaced with a solid plug, then you are in high compression load and shouldn't be operating at more than 1/3 power for a sustained period of time,
if you have a Dursley Lister. If part #147 is all that is replaced, you will be operating in low compression mode.
Maybe someone could post a picture of a COV plug so the rest of us can understand what's going on?
Stan