Did a service on my freelander last week and found the air filter was quite black with a sticky coating over most of the filter. Bits of fluff stuck to the outside of the filter on the black coating.

Also found the timing belt side of the engine was covered in oil when i removed the under tray, all over the bottom of engine on that side. Oil level was down at halfway on dipstick when it usually stays at full between services. Looks like a bit of oil is leaking at the top of the engine above the alternator but hard to see with the plastic cover on.

Is the oil leak likely to be related to the black sticky air filter?

Click image to enlarge

Is the vent at the bottom of the air cleaner box clear ?

I don't see how the two can be related. The air is drawn in from the wing, nowhere near your oil spill. The filter just looks to me as if it had a bit too long in situ. Was it changed within the last two years? It is only a bit more dirty than I would expect after two years, especially in a dirty (town?) environment. I think you are ok there.

The vent at the bottom had no dirt or debris on it, just clean plastic bits. Was supposed to have been changed 16k ago, my local garage did it.

Last few I've done wernt that dirty and didn't have and sticky residue on, just black dust.

I'm assuming the egr system recirculated the other side of the filter to burn off the oil vapour