Hi Neil,
haven't been able to do much for the past few days as the computer is throwing a fit and won't start up - and when it does it crashes again. Hopefully getting my new one tomorrow.
The Parkers are very confusing and it's hard to work out who is brother / sister to who. I am not sure if my Henry born 1848 is the son of your Mark, so I will have to do a bit more digging.
I have seen a tree that shows the following.
Marriage Mark Parker to Sarah Wilson 28 APR 1849 Ash, Surrey, England - this is on IGI.
Parents of Mark - Mark Parker born 1787 Buckland surrey. Elizabeth born 1797 Buckland surrey.
Children of Mark and Elizabeth are shown as Mark B 1851, James Born 1857, Job, B 1858, Andrew B 1866.
Not sure if this is correct but thought it might help.
Regards,
Jim