The CAA have put all their licensing information into a document called LASORS
here. If your CAA tickets have expired you might find you can still convert them as described in SEction A page 13. I'm a little doubtful about this route because the requirements seem to be aimed at a pilot flying with a JAR operator, which you are not
If you can't do that then you will basically be converting an ICAO ATPL to a JAA ATPL according to LASORS Section G page 5. Assuming you have more than 3000hrs on aircraft of more than 30T including 1500 pic you will need to complete form 680
here and send it off to FCL with the appropriate fee (about £80 I think). They will then reply that you need to sit two papers, Air Law and Human Factors, and take a skills test on the aircraft you are rated on, or in a JAA approved sim for that type if you can find one. The skills test is a combination IR and handling check.