You're right, I'm still struggling, so I'll take them parish by parish where I can add something (but I'll warn you now that I think you have a major reconstruction job on your hands!):
Chatteris - I think you are wrong when you say Robert was the son of William Leader from Cottenham. Chatteris parish register has
11 May 1794 Robert Leader base born daughter of Ann,
born 1 March 1785
11 May 1794 John Leader base born daughter of Ann, born
1 Aug 1787
11 May 1794 Samuel Leader base born daughter of Ann,
born 25 July 1789
"The father of the above children William BARTHAM carpenter at Broad Alder"
Bourn Parish register has the following baptisms
15 Aug 1802 Ann Leeder or Leecher, dau of Robert & Sarah
21 June 1807 John Leader son of Robert & Sarah
21 June 1807 George Leader son of Robert & Sarah
31 Dec 1809 Anderson Leader son of Robert & Sarah
(I think John was a delayed christening as he was age 50 in 1851)
The IGI has a marriage of Robert Leader to Sarah Story on 13 March 1800 at Kingston, which is the next village to Bourn. But as the Chatteris Robert would have been only just 15 at the date of marriage I think you have the wrong Robert. (15 was legal at that time, but unlikely)
The 1841 census shows Robert and Sarah Leader in Kingston, both aged 60 which means they should have been born 1776-1781
I think you need to be looking in Kingston around 1780 for a baptism of your Robert. Unfortunately Kingston baptisms are not on the IGI, but perhaps someone has the PR transcript. (If necessary start another specific lookup request for his baptism in a couple of days, or ask Keith on the Cambs Look up offers board to check it next time he goes to the Records Office)
(I have a Susan Leader b 1859 at Kingston in my own tree. She married my gg uncle William Surkitt - her first marriage was to John Bavister. I have just found she was the daughter of George chr 1807, so she was Anderson's niece!)
Whether Robert married in Kingston was connected to the Cottenham Leaders remains to be seen, but Chatteris is definitely a red herring.
I'll have a look at the Dickersons separately
Sorry if I've chopped down half your tree! You seem to have put your tree together from what you've found on the IGI - remember that Cambs coverage on the IGI is far from complete so you are running great risks of it being wrong, particularly when you seem to be jumping from one end of the county to the other and then half way back again. Reference to parish registers is vital.
Regards
David