Hugo, I agree- it is a great pity that the old chapel has fallen into such disrepair. As far as I know it is a scheduled ancient monument so you would have thought that Cadw might take an interest. Unfortunately they seem reluctant to become positively involved in monuments that are privately owned.
I think there have been four small books relating to the Old Hall and the chapel. The last one, undated, but probably from the early 1990s contains the final sentence: "The proceeds of this booklet will go towards the restoration of the chapel which is situated in the grounds of Penrhyn Old Hall". The accuracy of this booklet is undermined by the following howler:
"In 1760 Penrhyn Hall became the home of Dr John Williams, Archbishop of York and Lord Keeper of the Great Seal to King James l"
In reality, Archbishop John Williams, a notable native of Conwy, died at Gloddaeth in 1650. He had no connection with Penrhyn Old Hall but owned the original manor house on the site of what is now Penrhyn Castle!