Gladys; Like you I cannot find a baptism of Rebecca Cherry on the IGI in Dunstable, Leighton Buzzard or anywhere else in Bedfordshire.
I cannot find her on 1841 census or even a John Cherry
I did notice a FreeBMD Death Index of a John * Cherry at Leighton Buzzard RD vol 6 page 4* in Oct-Dec 1838; could it be him.
There may be more info on her1843 marriage certificate to William Chamberlain, you could send off for it or view the parish record entry at Beds Archives or Luton library.
On Genes Reunited there are 3 trees that have Rebecca Cherry born Dunstable, one most likely is yourself; have you contacted the other two people?
Regards John