I have a lightweight which has had a v8 fitted into and although not running yet I have noticed in many picture of series and lightweights with v8 fitted many people have added air vents in what looks like a position to get rid of some "hot air" can I ask are these essential or partially cosmetic?
Can't get mine to run at the moment so not really an issue but just wanted some advice in case I can getting running in the near future.

don't think they are essential, not on the rover 3.5 anyway

Never seen or heard of such a modification. Consider the amount of free space there is even in a lightweight engine bay, no undercover etc. cooling is via the cooling system. It's not an air cooled engine. What may be effective would be an oil cooler and or updated fan, but you'd only need these if you're doing a lot of offroading or running a winch.

I remember this same debate of on one of the saab forums for the tuned saab engines. The outcome was this:

cutting a hole in your bonnet to not always mean cooling your engine, or geting rid of hot air. The various air flow pressures have to be taken into account - for example, the addition of some sort of scoop under the bumper could create a low pressure area. A forward facing scoop on the bonnet would create high pressure air flow. air will flow from high to low, as per the water or voltage analogue.

I've probably not explained as well as I could, but the point being that a big hole in the bonnet doesn't always mean a cooler engine bay.

My lightweight (did have a v8 fitted) has holes cut in the rear of the bonnet on both sides

Cant realy see the point tbh but maybe it was thought to be a good thing at one time ?