From CAP804 Section 4 Part C Subpart 2
FCL.210.H PPL(H) — Experience requirements and crediting
(3)(c) Applicants holding a pilot licence for another category of aircraft, with the exception of balloons, shall be credited with 10% of their total flight time as PIC on such aircraft up to a maximum of 6 hours. The amount of credit given shall in any case not include the requirements in (a)(2).
So you have to do almost all the hours, except some of the supervised solo time.
FCL.215 PPL(H) —Theoretical Knowledge Examination
Applicants for a PPL(H) shall demonstrate a level of theoretical knowledge appropriate to the privileges granted through examinations in the following subjects:
(a) Common subjects:
— Air law,
— Human performance,
— Meteorology, and
— Communications;
(b) specific subjects concerning the different aircraft categories:
— Principles of flight,
— Operational procedures,
— Flight performance and planning,
— Aircraft general knowledge, and
— Navigation.
You are cross credited the first four and have to do the other 5 again.