Here is my solution for anyone who is changing from very thick oil to much thinner.
I took the diff apart as usual.
I removed the clear o ring as they are not needed anyway.
I then took every piece out from the differential, put a tiny bit of 30wt shock oil in the differential casing, mixed it up a little, waited a for an hour, then tipped it upside down onto a thick layer of kitchen towel, then used several cotton buds in the diff casing to remove the remains of the thick oil mixed with shock oil. (1 mil I think it was).
I then used tooth picks on the gear casing and every gear tooth to remove the rest.
It was rather time consuming, (about 2 hours in all!) but saved buying a new differential @ £40
Thank you for the suggestion Tak, I did not think it would, but it made the process easier.
I then refilled with my chosen oil (40,000cst) and replaced both the paper gasket and the large thin black gasket.