If your ATPL was issued in 2001 then it would have been a JAA licence that expired after 5 years together with all privileges in 2006. Earlier National licences had a 10 year validity. To renew the licence privileges you will need a valid Class or Type Rating and a Class 1 medical certificate. If the IR has expired by more than 7 years then you will be required to sit all the IR examinations and pass the initial IR Skill Test. JAA licences were accepted as EASA licences but the old UK licence was not and whilst they still exist have limited use unless converted to a UK Air Regulation licence, the current translation of an EASA licence.
In view of the range of posible options you should consider what you want to do and then go to an ATO and discuss the options and possibilities. To renew the existing licence or convert to another lcence will require an ATO or DTO. It is just possible that you had an old brown PPL prior to obtaining your ATPL and that could be a lifetime licence which could be reactivated with a class rating prof check and a Class II medical certificate or possibly a Medical Declaration.