Hi Guys.
I thought this fact might be a minor help to some of you others in the future? (I hadn't been able to find an answer from my many internet searches beforehand).
It is possible to remove and inspect both the "oil control valve" and the "oil pressure relief valve" from a type 3 (mine 1970) while the sump remains full of oil. Essentially zero oil loss from either when I removed them for inspection. I had wondered if there would be a complete draining of the sump considering there low-down position and had thought I would have to drain the car first. No need. Time and mess saved.

have the car on a serious slope, so the oil slothes to one side of the engine

You will probably loose the oil out of the cooler, i have had mine out on my hoist and you do loose a small amount.