[Solved] Kids are better off with us, but what can I do?

I apologise for the long post, but something that is constantly on my mind, and it might help if I could get some opinions from people who may have been in the same situation.
I am a father of 2 twin boys, aged 9.
I split up with my ex wife when the boys were 2, and although I looked into custody of the kids when we split up, as my ex is not abusive/an alcoholic/or done anything criminal, at the time it was considered by the legals that even if I paid the money and fought for them, it would be a waste of time, as my ex wanted the kids, and as she already had a house/partner lined up and I ended up moving back in with my parents for a time, the courts would inevitably go in her favour.
The boys have always been 'Daddie's boys', mainly due to the time and effort I put into the boys as babies (I did all the night feeds, looked after them when they were ill etc, and when we were going through the divorce I had the boys every weekend. Once she even abandoned the boys at nursery and rang me up at work in the afternoon on a Thursday to tell me I'd better pick the boys up, as she was on a train to London!)
Anyway, fast-Forward 6 years, and the boys have lived with their mother full time and their now step-dad (the man she cheated on with me) about 25 miles away from us (I have a new wife and baby on the way). The legal agreement in place is that I have the boys every other weekend (picking them up on Friday, taking them back Sunday night), but the weekends we don't have the boys, at some point I will get a phone call from their mother, complaining she can't cope and can I pick them up until Sunday night... so recently its been most weekends... (I think we've had 2 weekends off in about 4 months) Due to her shockingly bad parenting skills - she doesn't disipline them at all, and when their step-dad tries to, she undermines his authority by arguing in front of boys with her husband and saying that he's being too hard on them! Which all in all has resulted over the years in the boys hardly having any respect for their mother, zero respect for their step-dad, and constant 'naughty behaviour' updates all week from my ex, who clearly thinks she is doing nothing wrong.
When the boys are with us (my current wife is very supportive and brilliant with the kids and I would say they have mutual respect for both of us) they are as good as gold, and in general we all feel they are happier with us. In fact we know they are. We have had uncontrollable tears on the drive back to their mum's when they were younger, and now they are older they sometimes 'hide' when its time to take them back, also one of the lads has openly said he doesn't want to go back to Mummies, but knows he has to 'for school' and has admitted to his grandparents that sometimes he's bad at Mummies so she'll call Daddy and Daddy will pick him up at the weekend, when its not our weekend. Yes we have problems with fighting/arguing occassionally, but we deal with it when it happens and it never escalates into anything but a brief tantrum, which is over within minutes. When they are with us they sit down and do their homework/ we can take them anywhere and they are generally considered good kids by our families so we are happy to leave them with anyone (grandparents/aunties/uncles etc) and know they will behave. However, their maternal grandmother has recently asked to have the boys 'one at a time' only, as she finds it hard to cope with them together. We suspect again a lack of disapline here (wonder where their mother gets it from???)
From the reports we hear from my ex and his partner (who has recently taken to texting us all the problems going on over there) it is as if they are 2 different sets of kids, depending whose house they are in. Their step-dad is at the point of leaving completely, mainly due to the lads' behaviour, but also the lack of respect from his wife, and I can only imagine the state of their relationship at the moment, it seems to me, they are only together due to convenience/money!
She refuses to see sense and let the boys live with us full time, saying that we'll be 'taking her babies' away from her, although we all suspect there are monetary concerns on her part here as well (as I am paying child maintenance and have been all their lives).
As I've said before, there is nothing fundamentally wrong with my ex (which would hold up in a court of law anyway) - I.e. she feeds them good food/they live in a big house, in a nice area and go to a Church of England school. On the outside it is practically the perfect family.... to the world, she swears she will always put the boys first, but when she got a £3k tax rebate last year (and we were at the time talking about getting extra lessons for one of the boys who's more behind at school) she spent the money on a diamond ring for herself, and he never got the lessons because neither of us could afford them... (we found this out recently). Also we learnt recently that apart from nagging the boys to do their homework, she never actually puts the time in with them, prefering to spend the time playing with her rabbits and watching soaps... If we speak to her about the school situation, its all the teacher's fault/ they have taught them wrong and the school isn't helping them... and due to their behaviour, its everyone else's problem not hers - mainly taking it out on her husband (yes in front of the lads again!)
Is there actually anything I can do as a Dad? legally, to force her to give the boys up to us full time (obviously with weekend stays at their mum's?) At 9, I don't believe they are old enough to make the decision themselves, and of course I'm not looking forward to all the hassle this will cause, but at the end of the day you have to do what's best for the kids, not the mother!
Has anyone else been in a similar situation and what happened?