The A for Albert is not lower case. It is a way of writing in those days and the same for Mary Ann.
Have you looked after the birth for a marriage?
The Royal Sussex Regiment archives may hold clues.
http://www.eastbournemuseums.co.uk/royal-sussex-regiment.aspx
Try the record office too. http://discovery.nationalarchives.gov.uk/details/rd/c8e387ee-6864-480e-a39f-5713b2d71cbc
Thankyou for the input re. naming in those days... I'll go with Albert being his first name then. I guess the only other thing that troubled me was he was written down as Richard Albert Martin on Mary Richmond's 1919 marriage cert but of course this is likely a mistake from whoever filled out the form.
The 1901 census for the family in Brighton shows that there are children from the marriage to Thomas Back living with Mary, who is a laundry woman age 40 and born Ashurst Sussex.
Ref RG13 Piece 924 folio 9 page 9.
I assume you have this.
Yes I have this, I have her in 1911 as Back as you also found and as you eluded to afterwards I strongly suspect that they never married. But he must have been around for a little while as he was the one to go into the register office and fill out Mary Richmond's birth details...which rules out a fleeting romance which may have ended shortly after her conception.
There isn't much on offer in terms of Albert or Richard Martin's enlisted by 1898 in the Royal Sussex Regiment either.
Looking at Deaths from 1898 to 1919 Richard Martin died 1899 in Brighton (aged 35)...fit's in with not being with Mary Ann and daughter on 1901 census...