"You donot answer who Alexander married"
That's because I don't know! This is not my family.
An Alexander Marriott was on the the same page as Margaret Wilderspin in the GRO index at www.freebmd.org.uk
in the June quarter 1842 in Cambridge registration district, and as Alexander born c1823 at Cambridge (or Hertfordshire in 1861. or Essex Elmden in 1881 and 1891) appears with wife Margaret living in Cambridge in censuses I assume this is the same Alexander. He remarried between 1881 and 1891 to Frances*, and either of his marriage certificates should give his father's name.
Elmdon is only just in Essex, just across the county boundary with Cambs, and is 5 miles from Royston which is where his brother William was baptised.
*There's a marriage at West Hackney on 14 Aug 1881 between Alexander Marriott widower full age labourer and Frances Marriott full age widow. Fathers were Edward Marriott dec'd plumber and, wait for it, William Wilderspin sawyer. Witness was Frederick Wilderspin. So what was the relationship between Alexander's first and second wives?
George Marriott married Frances Wilderspin in 1867 in Cambridge and in 1871 they were living in Cambridge, George 50 born Elmon Essex (sounds suspiciously like Alexander's Elmden birthplace) and Frances 40 b Cambridge. George may have been another brother of William and Alexander, but as he predeceased William and if he had no children he wasn't mentioned in the 1893 will. Still working on George, but I believe his first wife was Mary Wilderspoon. They certainly believed in keeping it in the family!