Wow!
I am so very grateful to those taking the time to help me. So. From the info provided I have been able to establish that HDM born 1851 and Catherine Flora, are of the same family, are the right parents and interestingly, are both baptised at St Albans RC (along with 2 other siblings) yet are buried at St Anthonys RC. I have yet to confirm the birth records on GRO but did find a new spelling of McDonald on HDM's baptism - McDonold!! On both burial records the address is... Queen Street - the same Queen Street that is on HDM & Margaret's marriage cert in June 1849... Which is good. But...obviously throws up further questions!
So the timeline I sort of have now is:
HDM and Margaret marry 3 June 1849 at St Nicholas Church, Liverpool - which is a CoE!!
HDM a 'Shipwright' Both families in Queen St, Liverpool. (Parents home?)
Mary Ann is born 2 Feb 1850 and baptised RC in Stepney. (Where did you find those records from? Thank you SO much!)
30 March 1851. The Census shows Margaret and Mary Ann at 5 House 7 Court, Avery Farm Row, St George Hanover Square, Middlesex, England. Margaret a Dressmaker from Ireland. Is married, but no husband present. Mary Ann listed as having been born 'Pimlico, London.'
The 1851 Census also throws up a Henry McDonald, a Ships Carpenter born Lochgilpead, Scotland who is a Prisoner at House of Correction, Cold Bath Fields, Clerkenwell, Middlesex, England.
Yet we know, on 1st Oct 1851, their son HDM is born in Liverpool and baptised RC.
** I'm tempted to think maybe the whole London thing is a red herring with Mary Ann. However, the new found baptism records for her and the fact that I can find her with her family up in Liverpool and over the decades Census consistently states either Middlesex/London as her place of birth, makes me believe she was born there and HDM and Margaret did spend time there. The 1851 Census for both of them, ties in too. **
But why would they marry as CoE?!
Thank you everyone. It also helps to go over details when typing them!
M