Yes, taking the engine cover off is a good way of making sure it gets cleaned. When I bought my last vehicle, the engine had been detailed at the dealership. I did some work and had to take the cover off. Holy crap, that thing was dirty!! So yes, take the cover off so it'll all get cleaned.
Just for anyone else interested, cleaning an engine is not a difficult or scary task. I used to own a detailing business so I've got some experience here. Cover the electrics and air intake if it's exposed (think cold air intake). Don't spray a hard stream of water directly onto electrical connectors. Other than that, you're pretty good to go. Here's my routine:
After covering anything I don't want wet (with plastic bags and rubber bands/zip ties),
rinse engine bay and underside of hood,
spray degreaser on underside of hood,
clean underside of hood with various brushes, wash mitts, etc
spray engine bay with degreaser and working one area at a time, clean with various brushes,
rinse off and move to next section.
Cleaning a section at a time keeps the degreaser from drying before you get to it. It'll take some time to clean everything due to the nooks and crannies.
Once done, I use compressed air to remove standing water. Remove any bags you used to cover up stuff, then stand back and enjoy your clean engine bay.
Not a Land Rover, but here's my personal '86 F150 (I dressed my engine with a water based protectant):