It depends on what has been written into your own proposal. However, the usual policy, these days is what we refer to as the 10/50/50 rule.
This is where, of the overtime payment, an amount equivalent to 10% of your normal take home pay is disregarded (you get to keep that), then the balance is split 50 / 50 (half you keep, half you pay to the IVA).
A few proposals have basically the same arrangement, without the 10% disregard .. a straight 50 / 50 split.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
What does your own IVA say about this point rodmunch. As with most questions like this, the answer really lies within your own proposal. If you cannot find the relevant paragraph covering additional earnings, your IP will probably be able to point you in the right direction.