Thanks Mick, I chickened out with the large spanner after a couple of sharp hits, not being completely certain it was a left hand thread.
I did dismantle the bi-M strip etc, but could obvously not drain the residue fluid out. I managed to inject about half cup of hot melted wax through the control hole (duly tapped and screwed). The afternoon was hovering on 32 deg C , but much to my joy the coolant temp. now remained steady on 75. Testing with a wooden stick the clutch had now obtained full couple at idle. With engine stopped, can now feel slight resistance to a hand spin. The reason to remove the unit and drain completely is now dependant on a test how completely wax mixes with a small quantity of silicon over the temp. range. The wife has locked up the electric food mixer !!!. At least the unit actually works now, and does not leak.
Thanks for all your help gentlemen, I shall keep you posted.
I have never seen a station wagon version of the E220 over here Mick !
John (the tyke).