Well firstly, you don't have an ATPL (A) if it's a 'frozen' ATPL that you have. You have JAR CPL(A) with IR (A) and ATPL (A) theory credit.
Therefore you need to obtain a JAR CPL (H) with IR (H) and ATPL (H) theory credit.
Section D4 for the JAR CPL (H) requirements
http://www.caa.co.uk/docs/175/srg_lt...ection%20D.pdf
Section E2 for IR (H) requiremnets
http://www.caa.co.uk/docs/175/srg_lt...ection%20E.pdf
Theory required is mentioned in both sections, and yes you can keep both licences current at the same time.
Best answer is to write to CAA and get them if they can, to you write you an email with exactly what you need to do. They may not do this as they publish Lasors (the document I provided links for above) detailing what you need to do, but it's pretty confusing for those that don't read it often.