Op I have been looking for you at similar threads over on MSE and there have been plenty.
I would go over to the benefits board there, it seems if people fess up then TC take the overpayment back, things only get serious if you try and cover to CTC think you are lying and rule against your version of events.
There is loads of threads exactly like this where people split up etc.
I think the red herrings here are that one you say you have split up, but do you have proof? Was that a verbal agreement between yourselves as you seem from the op to still be doing things very much as a couple.
2nd red herring is he has worked away. Like I say many couples do this, does not mean they are single. The OH still supports the house and uses it as a base, that's a couple.
As an objective person and if I worked for CTC, how I would see it from reading your op is.......
You don't have a job, hubby goes abroad so you declare as single person, thus bringing in more money in benefits.
He continues to support you as a family, he is just on a stint abroad, he then comes back home to live, gets another job.
He then commutes during the week and returns home on the weekend, still supporting the family and still returning home.
You then have decided you are returning back to work in September so delcare youself now as "back together".
See what I mean? Pick out all the red herrings I would suggest you get advice as to what you are going to do, as in proper advice from CAB, to advise you on how to play it.