First post on this forum after much lurking
Over the past 3 months I've consistently been offered CE for £125 LHR-IST and £179 IST-LHR. £125 seems ok value for 80 tier points (up from 10 on a cheap ET fare). However when I went to accept the upgrade offer on the latest occassion (23rd Oct), I kept getting an error message saying 'can't process online, contact the call centre'. When I did that, I was told they couldn't do it over the phone as it was an online only offer. Frustrating! Ended up taking an AUP for £140 as I wanted the tier points.
Also, back on 26th August, I received an offer for CE from IST-LHR for £33! Nabbed that one. Assume because it was a full flight.