Usually I'd say CAP2254 has the answers (Military Accreditation) but that's not where your credit is coming from. In your case, you're bridging from your PPL(A) to PPL(H) theoretical knowledge. If you are only aiming for a PPL in each category and never intend to fly commercially then the reference you need is in Part FCL appendix 1. It states (via reference to FCL.215) you get credit for certain subjects when getting a PPL in a new category. The ones left are: PoF, Op Procedures, Flight Perf and Planning and aircraft general knowledge.
As to what to look at to study, there's plenty of books available - read up an Amazon or ask an instructor at the school is probably best.
Now if you do intend to fly commercially then it's potentially different. One assumes you flew on the frontline fixed wing. You get credit for the training before the exams - you have to do all of them . Once you have that, you can bridge to the 5 Heli specific exams to get Heli ATPL credit. However, I suspect that's all overkill.