Can't remember how she lost her tail - I know it wasn't intentional. She's no longer purple due to some weird time shift which brought her future self (a different colour) into the "present" and replaced the purple version of her. Yep, I know how that sounds :-)
The day after I posted this question I saw the episode on Sky where Trance loses her tail. They were under attack and something fell on it and it had to be amputated, thanks Smogster much appreciated. P.G