Does anyone know from recent personal experience what their DipTrans prep courses are like? What is tutor interaction like? What exactly were the pros and cons of the program you chose?
I only have experience of WLS and even then it was way back in 2006-7 and it was for their Certificate course, not the more advanced Diploma one. But I can say that the tutoring I received was first class. It's a pretty cheap course and although I expected it to be worthwhile, I did anticipate it being limited to "read the info, do the exercises, and we'll tell you whether you've passed or failed". Far from that, my tutor wrote lengthy comments on my answers and on the translations I did for each assignment. She praised the particularly good work I did, and gave reasons every time she marked my work down. As someone with many years of work experience but no university education, I found some of the sections rather difficult. I shone in all the actual translations, but she also expected rather "academic" answers to some questions and that isn't me at all, so I needed some hand-holding on the assignments too. It was made clear that she was available for more help, had I asked for it, by telephone or email.
Whether any of that applies today, and/or to the Diploma course, I have no idea.
For reference, I am a native English speaker, have an MA in Humanities (done in French, in France) and have lived in France for many years now. I have worked on a dozen or so (mostly general) translation projects.
I think they recommend at least two years of experience before attempting the DipTrans, so hopefully you're getting a lot of experience now. I know an awful lot of translators with a lot more experience than that who have failed at their first attempt. But good luck with your plans -- I'm sure they're achievable and it's a useful qualification to have.