StepChange do seem to be fairly 'laid back' so, hopefully, will take a sympathetic view, but, ultimately they could cancel the IVA for constant mis-use of funds. For instance, creditors have agreed to allow you, say, £300 per month for food and household items --- if you are spending, say, £200 a month of this on gambling, then they could rightfully reduce this allowance to the £100 you are actually spending on the agreed items and expect the 'spare' £200 to be added to your IVA payment each month. Most of us switch around the odd tenner here and there, but the amounts you are talking are a different matter.
Also, if gambling played a part in getting you into debt in the first place, still doing so could threaten the viability of the IVA if things are going south again. Creditors want to see you climbing out of debt, not digging further in. If you look at it from their point of view, you are gambling with their money and they will not be happy about that.
You have only fallen off this particular wagon for three months or so, hopefully, they will look on it more positively if they see that you have recognised the problem and are taking active steps to nip it in the bud.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014