Hi all.
Update.
After 77 days of hanging on the ramp in Lexus garage, I got my baby back.
The engineer working on my car said , that in his entire career in Toyota\Lexus service(22years) he never had such a problem. The gearbox and all associated parts which had contact with transmission oil were replaced for brand new according to Lexus technical department recommendation with hefty bill over 17, 000 British pounds just for the parts. Everything what been uninstalled from my car had been sent back to factory for post mortem examination. It's not likely they will share the knowledge about this problem unless it's necessary to introduce any critical improvement affecting other cars with this gearbox. This would lead to recall action .
Anyway, the car was on warranty, it costed me nothing, got new transmission and I enjoy my ride again. Two and a half months waiting time is mostly awaiting for these associatiated parts which are specific to RCF's and not available on demand.
One thing I can realise after the replacement is - the box is working quicker, with no such a long lags in reaction for pressing the throttle. I was driving the sc430 for 7 years before, with both 5 and 6 gearboxes and response from them was much better, what was making me bit concerned. Now is better, but on normal mode still bit sluggish. Sport mode is ok.
Got another appointment in the garage in 3 weeks time for transmission oil service.
Hope that's the end of problems, now got to replace axleback exhaust and the infotainment interface to enjoy my beast even more.