I can tell you, it's going to be Lucas hot-wire EFi. That's good because it's so much better than the old flapper system.
If you can stand the cost, the Range Rover classic manual has a really good section on the Lucas EFi and how to test all the components with a multimeter, mostly.
It's worth doing just to be sure all the connections have been made properly. For example if the road speed sensor has been ignored this could give you a problem.
The EFi regulates idle speed with a stepper motor in the housing at the back of the plenum. It's supplied air via the rubber hose that loop behind the throttle cable and butterfly. The throttle butterfly itself closes fully at idle, if this has not been set up properly the EFi will struggle to control the engine and this could be what's happening. or the stepper motor could be stuck.
A faulty MAF sensor (hot-wire) might screw up the mixture but TBH mine will start and drive with it missing completely, perhaps this is easier to deal with than a faulty one.
Does it have lambda sensors, again these could be Lying.
The EFi also know when the heated screen or airconn are on and steps up the idle speed to keep the alternator charging.
Tiy need to chck the installation carefully as it's likely on a retro fit that something has been missed or become faulty.
