One of my accounts has not been able to keep up with payments of NICS, has contacted HMCE and arranged payments - so that bit is fine.
Where can I put the amounts owed, so that it shows on the system that it is owed (similarly to a purchase invoice). Normally it would just show as a payment out when it has been paid, but I don't want the client to think he has more money that he has!
Assuming your client is a soletrader and not a limited company the NIC's would not be a business expense. However if you want to show the eventual liability you could post a purchase invoice for the amount owing and choose drawings as the nominal code. This would keep it out of the P&L but would show the liability in the B/S.