The TL4 is a piece of s**t, mechanically.
Let mine get properly fucked up before I took it in, was under warranty though and I told them exactly what was wrong. On inspection they said it had a "Serious Malfunction", guy said there was bits of metal everywhere and it was chewed to bits. Even the updated version was problematic, asked for exact part numbers of each when I had mine fixed.
Some people have managed to get theirs sorted out of warranty I think but doubt they are under any obligation to fix FOC. Think you've done well to get 60-odd % off. Although £600 for fitting a gearbox, are they that expensive?