What is it and whys it leaking.

Really what the title says see circled item in picture. Unsure of the name or what it does. Noticed this has a rivited cover and when engine is running you can see a gap which is allowing oil to slowly cover the os of the engine bay. Been doing it a while just never found where it was coming from.
Does anyone have a fix?

vacuum pump ,you could bolt or re- rivet it

Yep, brake vacuum pump. What happens is the rivets holding the end plate stretch or snap entirely allowing the lubricating oil to spray out. It needs changing pretty quickly as if it fails entirely you'll lose brake assistance and believe me a D1 takes a LOT of stopping - without the vacuum pump you'll be doing a very very leisurely loss of speed even from a walking pace. I can tell you that first hand as mine went 2 weeks ago and stopping from 10mph took 40yds or so - scary!

You can do a short term repair by removing all the rivets and replacing them with bolts and using silicon to seal the end plate. I did this last year, but mine has now failed entirely so I would only recommend replacing the unit entirely although many people have found the repair to last years.... your choice.

I'm replacing my pump today as it happens - got an Allmakes one for around £50, a genuine Wabco one will cost you £100-120. Don't forget the gasket

Iv always had a drip under the car on that side but because it covered the chassis leg in oil it was hard to tell where it came from was only when it was running i seen the pulsating gap. Ill drill out rivits and replace with bolts for now. Is the unit full of oil?