Hi Rose
You are quite right about the discrepancy. I was so excited to find these entries I didn't stop to look at them carefully. I am now very unsure about this birth, especially as it would mean that she was 44 or 45 when her youngest child was born. Not impossible, though.
My reasoning in looking at it was that the names Maria and HANAM are not that common in England, however according to census and FreeBMD there are others with the same name.
However:
I note that she is listed on the two census entries I can identify as being older than her husband - perhaps she put her age down a bit so she would not seem quite so much older than her husband. The census information is what people chose to tell the census taker, and he may have mistranscribed anyway - unlikely in this case as she is consistent over the ten years. That would be a fair age gap though - 13 years.
There is an outside chance that the baby Hannah born and baptised (St Martin in the Fields) in 1806 may have died and another child born to the parents may have been named the same - this was not uncommon but is a less likely scenario. Unfortunately civil registration did not start until 1837 so there will be no official record of a birh in 1906.
The marriage certificate should give her age, her parish and her father's name and occupation - assuming the informatin given at the time was true and correct. Unfortunately the name of the church as given is also the name of a registration district so the marriage may have been in any church in that district which covers Belgrave; Hanover Square; Knightsbridge; Mayfair; Mayfair & Knightsbridge; St. John; St. Margaret; St. Margaret & St. John; Victoria; Westminster St. John; Westminster St. Margaret.
You would need to order it from UK. This information is from the London resources on Rootschat
You can order BMD certificates directly from The GENERAL REGISTRY OFFICE (GRO):
http://tinyurl.com/5wn3hSorry I have no idea of the cost etc.
Hope I haven't led you too far up the garden path

Judith
I'd like to know what others think? - Jenn?