If you never obtained a JAA IR within 36 months after passing all the ATPL exams then you will have to do some theory exams again, however it's not all 13 exams.
If the UK issued you JAA CPL(H) at present you will need to sit the 7 IR exams, once you gain a JAA IR you would have an ATPL(H) theory credit.
This may change in the future at present the UK CAA are still using amendment 3 of JAR FCL 2, when or in deed if the UK CAA choose to implement amendment 4 of JAR FCL 2 you would then have to sit 5 exams at ATPL level.
You can down load a copy of JAR FCL 2 from the JAA website and you can down load a copy of lasors from the UK CAA website.
Get the CAA to confirm this is writing before you start any exams.