Hi Callum
Re the dna match and their ancestor per your post
"I have used the leads method for a 2nd Cousin Match and I know he is the descendant of William Collett and Charlotte Robbins from Worcestershire, England and this couple had about 9 children and none of their descendants I could track marrying into the Bennett surname and having a child with that name at any point and all shared matches with this person are 4-6th cousins or 5-8th cousins that share that 1840s couple or their grandparents".
I descend from that lineage, and I have a broad tree spanning siblings and cousins so I can check my tree for any marriage to Bennett if that's your missing link.
William and Charlotte are my x2 great grand uncle & aunt. Williams brother James is my x2 grt grandfather.
I'll take a look as I have traced a lot of these ancestors and their offspring.
This post is over a year old, but hopefully you'll see this reply

Kind regards,
Michelle Collett