I pay £600 a month to the ex, but decided to stop £200 from the maintenance to help towards the kids holiday of 12 days, I've now had solicitors letters demanding I pay up, what could be the outcome in court,

Well, I'm not from there but I do know that in the U.S. support (maintenance) is calculated to include the time spendt with the non custodial parent. Therefore, the payments wouldn't change the time the child(ren) are away from the custodial parent is already figured into the amount owed. So, at least over here you would still owe the entire 600 not 400. That being said if you don't pay up the court would order it to be paid anyway possibly with interest depending on how much time has gone by. You should look at getting your own lawyer closer to you to tell you the exact laws of your area.

And you can't just "decide" to change the payment without a court order saying you are allowed to. Again here in the US, payments don't change unless the court orders them changed, no matter were the kids are.