Hi Rachel - sorry for the delay but the forum has been out of action for most of the evening.
I always find it a little dangerous to work salaries on an average basis, as this does not take allowance of weeks where you will only receive basic pay. However, if they have built in overtime to the general calculation, which you suggest that they have, he will only need to account for 50% of his earnings in excess of the £980 per month.
I suggest that you work on an average weekly figure of £226 - and set aside 50% of all earnings he receives in excess of this in a separate deposit or savings account. If there are some weeks when he earns less than this, you can release monies back from the savings account, but in this manner you ought to have the money available to pay over the the IP at the end of the year.
Hope I have understood your post properly.
Regards, Melanie Giles, Insolvency Practitioner