You also need multi-crew experience to exempt yourself from the otherwise compulsory 650 hours of JAA ground school (only 150 hours as I recall?).
Suitcaseman, not entirely correct... conversion requirements vary depending on license held and experience. For the theory exams, you are exempt from any groundschool if you hold an ICAO ATPL. As far a flying goes, you cannot hold a JAA ATPL unless you have a type rating on a Multi Pilot aeroplane. JAA has a defined list of "multi pilot aeroplanes" (MPA).
If you hold an ICAO ATPL, are type rated on a MPA
and have at least 500 hours on type, then you can convert your ATPL by doing a standard 6-monthly Prof Check with a UK CAA inspector to observe.
If you hold an ICAO ATPL, but are not type-rated on a MPA, then you can undertake a
full typerating course at a UK training organisation. In this case the final flight test qualifies for license conversion and again, has to be observed by the UK CAA.
MK are now a UK accredited training organisation and are actually offering the guys a pretty good deal- compared to the average £20K charged for an A320/B737 rating by other schools- and with a job offer attached to it.
Cheers/JD