Coolant leak - no obvious external signs of leaks. I have read a little about a gasket between the intake manifold and the block on V8's being a problem, especially on aluminium blocks because the bolts can't be as tight as on a cast iron block. I'm hoping this is the case and it isn't the head gasket or cracked block. The oil level is spot on and the engine is very smooth - also no water contamination apparent on dipstick. Any help greatly appreciated.

It would be preferable to have a real, trained and experienced BMW mechanic to take a look at your engine and diagnose it properly.
Guessing is not for BMW's for mistakes can be very costly.
It is true that the valley pan gasket can cause problems. For this you would have to take a flashlight and look in-between the left and right cylinder banks to verify if there is water beneath the intake manifold.
You could also bring the car up on a hoist and see if water leaks down the back of the engine particularly down the bell housing holes.
Or it could be a tiny water pump leak due to a bad bearing which makes the shaft wobble. Or worst case scenario a bad cylinder head or block or head gasket.
In fact it can be a lot of other things big or small.