According to CAP54, which is, I believe, still valid for UK licenses, your ATPL exams are valid indefinitely for your ATPL (assuming you have a CPL issued).
However, since the authority won't issue a UK licence after June 2002, it's not clear to me what would happen after that to UK CAA exams.
It seems to me that, again according to the CAP54, they will still be valid indefinitely for, for example, the addition of an IR to a UK CPL, but that you would have then to get onto the JAR scheme for the ATPL issue - for which the ATPL exam validity times are different.
For JAR, the exams remain valid for 3 years (I think) or for 7 years after the last validity of an IR.
So..., my interpretation so far as that you need to have a UK CPL before June 2002, you can add an IR to that at any time, and can then convert onto the JAR scheme and be able to get the JAR ATPL issued up to 7 years after you let the IR lapse.
I'm hoping my interpretation is correct, but I haven't had it confirmed. When I rang the CAA to confirm it, the woman I spoke to didn't seem able to say anything beyond the JAR rules and that CAP54 is becoming defunct. She seemed totally confused by any interraction between CAP54 and JAR rules.
Has anyone else tried to confirm this?