Just had another go; a complete fiasco. Tried turning in earlier, with greater speed, but what happens almost every time, is I hit the apex too soon and thus get the line through the corner all wrong. At Stowe corner and only there for some reason, I would provoke a massive rear slide, at best shedding a ton of speed, but frequently ending up spinning. Have to devote some time to the upcoming race now, so have to put a hold on this for now. Thanks for your efforts Hristo.
It sounds to me like the issue is mostly that of not trail braking properly and/or turning in the right moment. Perhaps you're lifting off the brake pedal too soon or too much before or during turn in. Difficult to say without being physically present, as you said.