There was Elsdon activity in Ramsden Bellhouse but having checked 1820 - 1830 for you, I can confirm a George Elsdon is not amongst them. It may be that he grew up there but was born and baptized in another parish.
Have you tracked him through the census decades? Perhaps you'll find he varies his place of birth on them.
Can't help with Eliza Marsh I'm afraid. I haven't got Brentwood registers.