Unfortunately, recently, many IPs have been re-interpreting the terms, but, basically, this is how they were meant to be operated:
Overtime and bonus payments fall under what we call the "10%, 50/50 rule": On a monthly basis (though some will do it at other agreed intervals) extra income is to be accounted for to the IP. The first 10%, based on 10% of normal, agreed, basic pay, is ignored ( you keep that). Then the balance is split 50/50 with the IVA.
So, say your normally take home £2,500 a month and, one month, you get £400 overtime. Deduct the 10% of usual pay - £250, leaving £150 to be split 50/50, so £75 each ---- the IVA gets £75, you get £325.
Permanent pay rises are dealt with differently: The way my IP did it, and which used to be the norm, was that the payrise was only taken into account at the annual review following the rise and, one month after that review, the regular IVA payment was increased by 50% of the net increase in basic pay. These days some IPs will institute the increase as soon as you get the rise, by the fictional introduction of an interim review ( if you twist and mangle the words and spirit of the terms enough it can be justified, legally, if not morally ). But, in any event, the increase should not be more than 50%.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014