I'm pretty sure it's five hours plus exam; the same as if you didn't have a rating at all. I had a similar query with a South African Jet Ranger rating and was told by the CAA I would need something like 500 hours in that particular aircraft type to "convert" the licence.
However, I don't know anything about 15 hours for instruction - I would have thought if you have 20 hours already, you should be able to do the rating in minimum time and that if you had the rating, you could instruct in it - provided your instructor's rating is similarly valid.
Cheers
Whirls