Hello Jim1,
I live about 5 miles from Whitford but I don't know of a Mertyn Hall. I did ask in work and got little response.
I think that this may be part of the Lord Mostyn Estate in an area known locally as 'Downing'. Follow the North Wales Coast into the Dee estuary and you will the village of Mostyn. At right angles to the coast and inland you will find Whitford. Downing is in between and it is just an area of farmland, woods and countryside.
Try the Flintshire Record Office at Hawarden on 01244 532364 and let me know how you get on.
Thank you for the compliment. [
