You need to speak to your IVA company - if your payments are sorted out weekly it may well be that it is sorted out weekly but they will be able to help you.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
Weekly will bring in higher contributions to the iva if OT is not regular. You could ask the iva company how it's to be worked out without suggesting weekly. Ask by email and if you get monthly as your response, happy days.