If you want a UK ATPL you have until 30 June 2002 to apply. You will need a valid medical and a current M/E JAA/UK aircraft rating but I don't think the IR needs to be current so long as it is not more than 5 years out of date, in which case you would have to do another initial IR Test anyway.
If you want a JAA ATPL you will need the 500 hours MPA a valid medical and a current multi-pilot aircraft rating JAA/UK this would include IR validity.
The validity of your ATPL exams for a JAA licence is for ever so long as you maintain the CPL and do not let the IR lapse by a period of 7 years or more.
Best Tip, get a UK ATPL by June this year.