Maybe -- it's unclear -- you don't need slips from previous visits. Here is the email reply from my query to the IRIS office:

Dear RichardInSF,

Thank you for your support of the IRIS system.

You will be pleased to know that your current IRIS registration actually expires on 29th October 2008 and not July as you expected. As you are already successfully enrolled on the system, you do not need to re register but may update your record by visiting any IRIS Enrolment Station prior to October this year. The Enrolment Officer may ask you some questions about travel frequency and the usual BIA checks will be done at the same time. The whole process should take less time than the original registration. Please ensure you tell the Enrolment Officer that you wish your record to be Amended.

You do not need to pay for any IRIS service - it is free of charge.

You may find it useful to print this email to take with you as reference if you do decide to update your record on your next UK visit.

Kind Regards,
IRIS person

Gives me hope, will try when I am there next in a few weeks.