firstly you might want to try
http://www.familysearch.org/ for Samuel,s father. He could be listed on the 1881 Census. I did try for you but it's down again

Should be ok later today.....just keep trying.
I've also looked in the NBI 2 ( National Burial Index 2nd edition) and there are a few Cusworths buried in Doncaster but none that totally match who you are looking for.
There is a Martha Ann buried 29th Nov 1928 aged 67 at Hyde Park Cemetery, Doncaster. She's the nearest match.
It looks like you need to get to see the parish records then once you have a possible birth for Samuel in 1905 you can order his birth certificate. You'll then get details of his parents and can look up their marriage.
It's possible that FreeBDM have not fully transcribed all the records for Doncaster as yet.
If you ring the library they might offer a lookup service it should only cost you a couple of quid.
Good luck
Fitty