Thank you so much for taking the time to answer and advise re illiteracy. I was not particularly looking for official name change documentation but rather anything other than BMD records that might be of help (like was the first ancestor to change his name, Thomas, in trouble with the law/running from something/on the 'wrong side' in a civil riot etc. Just something to indicate he might not have wanted to be found! The family come from Essex and I'm going to spend a week there in May and wanted to use the time effectively by having some document names/types to look through (you can probably guess from my use of genealogy language that I'm a beginner!).
Anyway, what I have is that Thomas was born in July 1792 in Lawford, Essex to Edward and Elizabeth Merven. I got this from the parish register of his baptism. He married as a single man a woman named Ann Powell on 7th Oct 1815 in Little Bromley. He gave his name as Thomas Maythorne. Then in the parish Baptism book the entry for their first-born Robert states parents Christian names as "Thomas and Ann late Powell", and surname as "Maythorne alias Marven".
Meanwhile, Thomas's brother William (found in parish records, same parents etc) married Ann Powell's sister Mary Powell, and had a child Sarah in Little Bromley whose Baptism record states parents Christian names as "William and Mary late Powell" and surname as "Marven alias Maythorne". William and Mary continue to use the name Marven from thereon. Thomas changes his name back to Marven sometime before he died in Little Bromley in 1823 (parish burial records). One of his sons, my direct ancestor, Edward was baptised as Marven then married under the name Maythorn to Francis Cole Daniel - the reason I give her full name is it has helped to identify that the family used both Marven and Maythorn surnames in various records. This is becoming soo long to explain, but suffice to say I've found Edward, Francis Cole and their 6 children in various census records under the surnames Marven and Maythorn. I keep wondering if they are two separate families but have found proof over and over of kinships within the records that prove they are one and the same.
So, if you've made it to here without totally losing interest or the plot, what I am trying to do is put some flesh and bones around the families of Thomas and William, and of their parents to try to understand why they used both names on various documents and actually had written into the parish registers that they were using an alias.
Thank you for reading this.