That's great discipline from aguise - I wish you were my client!
In my practice we review this at quarterly intervals to ensure that clients do not get nasty suprises at the end of each year. If this has not happened in your case, and you have earned overtime during the last year, then you will be required to pay over the 50% in a lump sum on the basis that it should have been set aside for such purpose.
Do make sure you let your IP know about any increased expenditure, which firstly must be borne by your 50% share but could impact upon the amount they eventually demand.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk