Thank you all again for your input & messages.
Just to clear up a few things, I don't mind that the boys stay at the dad's new GF house, so long as I know where that is, as far as I was knew, they were at his flat, a stone's throw from my house...I do not know where the gf lives, however, she is a friend of a friend and if push came to shove I'm sure I could find out the address, but think really I shouldn't have to do this and ex should have mentioned they were sleeping at GF house, after all, they both came to collect the kids (first meet with GF), and her being a mother herself should understand this, I'm sure she knows where her daughter is staying when with the father!! It was quite distressing to find out that my kids weren't where they were supposed to be.
My ex hasn't been directly violent to the kids, he's smashed my house up and stuff, which the oldest has witnessed,which is plenty enough...I think my son's main worry was him getting into trouble for telling me where they were (or weren't)...or having to come home early - he assured me he was ok and having a good time.
I also have a friend who's ex refused to give contact details, he also was made to pass these details on by a judge, who thought it was ridiculous that the mother was "not allowed" to know where her children were sleeping.
The ex is due to collect them again a week on Friday, I will put it to him for the address thing (No point even trying to talk to GF, he's at her side 24/7 and doubt she would be able to talk to me), if he declines the info again I will withold access til I've seen a solictor, who I'm sure will agree since the history of the case!!
Very very interesting to read everyone's opinion on the matter...thank you again! (PS, this is my first post and haven't got used to the abbreviations yet)