Alternatively,
As a commercial pilot, am I right in assuming that my JAR ATPL automatically becomes a EASA ATPL from 17th September? Therefore I do nothing??
Yes, it is 'deemed to be' an EASA ATPL.
Is it valid indefinitely, unlike the 5 year JAR Licence?
No, it is valid until the expiry date that is entered in the licence (5 years after it was issued). When the JAA licence expires, it will be replaced with a real EASA licence that is valid indefinitely.
Type ratings...I heard that it is going to be similar to the FAA in that type ratings stay on your licence forever but i was then told that they are only valid for 3 years.
An EASA type rating is valid for one year, as is a MEP class rating. A SEP class rating is valid for 3 years.