Unfortunately the Rover V8 head gasket is fairly thin in it's cross section at the rear of the engine block. I've seen a number of motors where there's dried up ooze of coolant weeping between the head and block at the rear of the motor. Obviously this is hard to see when the engine is in the engine bay. You might need to use a mirror to check for this.
The other place where a DII V8 would leak coolant is the throttle body pre heater. Check the two hoses - if there's crud under the throttle body connections, that's another source of coolant leaks.
The other common place is the sides along the radiator - pop off the plastic radiator shroud and shine a torch down the inner sides of the radiator. Spray some water from a misting pump and if you start to see a tell tale sign of orange, you've got a radiator that is leaking....
Regardless of where the leak is coming from, or how quickly/slowly your coolant bottle is emptying, get it fixed ASAP otherwise you'll end up with much more expensive repairs (head gaskets, slipped liners, etc). The old adage, a stitch in time saves nine is so appropriate for these motors!
