Thanks to Bertie for finding the family on the 1841 census, I thought I had it all wrapped up nicely with the births and deaths of the 2 younger children (leaving young John to travel with mum) and the marriage of the parents. I believe(d) Mary Ann was Flaherty by birth and this would tie in with the forenames of siblings who lived in the same street or area. HOWEVER I have just seen the below on a Rootsweb board and without buying the birth certificate I can't say for sure that the John GRIMES I have found on the BMD is mine - there is only one John GRIMES born 1837 in Manchester. What next?

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Subject: Re: [LAN] William and Mary Ann Grimes marriage circa 1836
I found the following . Wonder if its him and this is your chaps mother ?
Lancashire Birth indexes for the years: 1837 Surname
Forename(s)
Sub-District
Registers At
Mother's Maiden Name Reference
GRIMES John Ancoats Archives+, Manchester Central Library MCNALLY ANC/1/32
Ancoats is only a short walk to the cathedral.