Thanks for the reply. I am leaning towards a sticky relief valve being the culprit. What I dont understand is how/why the valve is being forced open and how, if it is sticky/stuck, it apparently closes when I change the oil filter.

I am going to have a closer look at my bilge to see if an in boat, no sawzall, droping of the oil pan is possible. If not I may borrow your sawzall to do the job!