I believe you will need an approved school to back your application to sit the exams. CATS in Cranfield is extremely cheep at 999 £, and has a god rep (do a search for CATS here on pprune).
www.cranfieldaviation.com. Question bank included.
Apart from the 14 ATPL exams, you will need a customized training program tailored to you experience, made by an approved school and approved by the CAA. And then of course the skills tests for CPL and ME/IR.
BUT! Although we all like to give advice, few of us have actually converted from a non JAA license. So you MUST get the correct information directly from the horse’s mouth, which is from the CAA, if in the UK. Don’t start down a road that will get you nowhere or cost you more, because of some hearsay. (And don’t think that the CAA speaks for the national authorities in any other JAA country. They all have there individual interpretation of the requirements).