At Erddig Castle I was told that the double 'd' is pronounced like 'th' but firmer, more emphatically than in any English word I can think of. So for Erddig, think of how you say 'earth', but shorten the 'th' and you will be somewhere near the right pronunciation. It is actually said as 'Airthig'. Hope this is correct - it's what I was told.

I believe the initial 'C' in Cynddylan would be pronounced like a 'k'. So you would have 'Kinthillan'. Now someone can tear me apart!