I studied Japanese at university, went to Japan and a book which I helped translate has been published albeit with a different translator's name. I passed an agency test with one of the best agencies in the world and had a job translating patents and did some agency work.

However, I stopped studying Japanese due to splitting up with my Japanese girlfriend and being offered employment outside of Japanese and also my study of Turkish and Arabic.

I have now adjusted myself to my new job and want get back into learning the language which I loved!

Has anyone got any advice for relearning a language.

Hmmmm. I've taken more long breaks from the language than I care to admit...

If you still have an Anki/memrise/etc. deck, then reviewing that is certainly worthwhile, especially if it is in the form of sentences and not isolated vocabulary. Re-reading materials you used to be able to read also helps, even if it's just re-reading your textbook dialogues. Essentially, the majority of your previous knowledge is still there you just need to trigger the recall of it to get it back. For that reason, it's not sensible to reset the times on any SRS software you use - fail what you absolutely can't recall and pass what you recall even if it takes a struggle.

After just a bit of review to get the gears moving again, then just start engaging in whatever form of the language you enjoy most. It'll probably take more dictionary look-ups than it would have before, but it should come back quickly.

I don't think it's worth re-studying grammar principles and such - what you have embedded in your experience is worth more than what any beginner or intermediate textbook can say. Studying grammar as a set of rules is really only useful for initial understanding and fairly useless on getting back up to speed after a lapse.

As someone who recently came back from learning Japanese after a 4+ year hiatus, I can tell you what I am doing. First, I know there will be holes in my knowledge since the last time I even touched Japanese so I took the J-CAT to help me determine where I am at in that very moment. I wasn't very thrilled about my score, but now I know what I would need to work on (except speaking and writing as the test does not have these categories).

Next, you can look back at some old material you had done in the past and see if it is still familiar to you. Since you were a translator I'm sure you don't have too many holes but you can once again see how far you have gone in the language. I had plenty of holes but at least I'm proud enough to say that I at least didn't forget kana lol xD

Finally it is probably best to ease into the study aspect of the language and do more fun aspects at first. You don't want to burnout at the begging. I try to keep my studying to less than an hour and just try to do fun things in Japanese like read manga, listen to music or watch shows. Some might disagree with the small study time but you can eventually increase it over time. For me, doing 45 minutes/day of studying is enough where I learn new things without being overwhelmed.

I came back to studying Japanese after a several year hiatus, and I've been reading a lot and studying vocab with Memrise and Anki. For a while I had to study words I'm sure I used to know but just had trouble recognizing. It sounds like you were at a high level before, so I think reading would be a great way to get back into the language. You'll see where your weak points are and can determine what kind of study you might need from there. Reading/Listening to audiobooks with transcripts would be great as well--in fact that might be the fastest way to bring back your passive language skills.