Thank you so much for your response. Since my initial posting I have carried out a lot of research on the family, including going to Tredegar. I checked the church, but there is no-one relevant buried in the family area and the rest of the church is carpeted, so I couldn't see anything more. The graveyard is very old and uneven, with many collapsed graves, and most of the headstones are illegible. I then went round Tredegar House, carrying an early picture of John Morgan (Brazil), trying to spot any resemblances in the paintings, but nothing jumped out at me. I managed to see the house rented by William jnr in the Forest of Dean, and the owners very kindly allowed me to look at the oldest part of it - only one room, as it's now a care home.
The only other information I managed to find was in a book by Houston Stewart Chamberlain. He was very disparaging of the Morgan line, describing William snr as a 'horse keeper' in the Vale of Evesham and inferring that both he and his son were spendthrifts, with reference to the bankruptcy case - about which, again, I have found no details.