One step at a time!
You can find MAry's maiden name from Bertrams birth cert.
Hopefully that will lead you to George and Mary's marriage - and that cert will give you George's fathers name and occupation.
As George was born pre 1837, i t will be a case of searching parish records for a christening. With a place of birth from the censuses (have you found the family in 1881 which is free to search as a starting point?Though best to find on all to see if consistant with yr and pob) and a fathers name, you will have more chance of finding the right record.
Some Kent PR's are online on the cityark site, sorry not got full address to hand, but it will come up on a search. Dont know if this covers Bromley