Hi Mercel
I found the marriage record on FREEBMD:
http://freebmd.rootsweb.com/The marriage cert, as you know would confirm Robert & Sarah's father's names and occupations, and poss have an address of Robert & Sarah at the time & witness' names to assist. And the birth cert of your Gt Grandfather Thomas or one of his siblings, would confirm his mother was Sarah NICHOLSON. Until you have at least those, everything I'm suggesting from this point, is just suggestion

though I have to say, it's looking promising!!
You've said you've been given information already on RC - does this include all possible Census information from Robert going forward after 1861, after Sarah died, since there might be clues regarding his parentage/siblings....?
However, on IGI is a possible christening:
Robert FORSTER Chr: 7 Aug 1825 Tanfield, Durham, England
Father: Thomas FORSTER
Mother: MARY
The IGI also has the following extracted christenings in Tanfield to parents Thomas FORSTER & Mary - they could be siblings to your Robert:
John 15 Nov 1818
Dorothy 24 Sep 1820
Thomas 20 Jul 1823
George 30 Sep 1827
Edward 21 Feb 1830
On the IGI is an extracted marriage where the forename & surname of the marrying couple are the same:
02 Jun 1817 Tanfield, Durham
Thomas FORSTER
Mary FORSTER
On the 1841 Census, considering age rounding, there were at least 11 possible Robert FORTSER - and this did not include name variations (Robt/ FOSTER/) etc all over Durham......
On the other hand, there were not so many Dorothy FORSTER - the most likely one did transpire to be in a house with a Thomas & Mary of parental type age & right occupation, and a Robt. born abt 1826...
1841 - Chester le Street DurhamHundred: Chester (Middle Division)
Source information: HO107/300/1, Folio: 36
RD : Chester Le Street Sub-RD: Lamesley ED: 2
Address: Township of Birtley
Thos FORSTER 55 Cartman
Mary FORSTER 50
Dorothy FORSTER 20
Robt FORSTER 15Next door to them was :
Geo. FORSTER 45, Cartman
Eliz. FORSTER 40
Thos. FORSTER 15, --on? Roller
Dorothy FORSTER 15
John FORSTER 5
Marg't FORSTER 5
Isabella FORSTER 1
ALL born in County Durham.The names of this 1841 family certainly echo the names of your Gt- Grandfather Thomas and his siblings, and the occupation is certainly right!
Cheers
AMBLY