Hi Eileen,
No worries, I work full time too, so I know what it can be like trying to squish in the research time

I would love to find a link between the two Mensforth families in Adelaide too, did anything come up in the letters that John Mensforth wrote? I would love to see copies if you know where I can access them or if you have a copy?
In my hunting around with this family I have come across the following in regards to your John Mensforth:
Another Rootschatter found that the National Archives Records state:
Item reference WO 97/649/82 (1818-1841)
JOHN MENSFORTH Born KIRKHEATON, Yorkshire Served in 51st Foot Regiment Discharged aged 41
I found some baptisms in Kirkheaton to this family:
Marriage Edward Mensforth & Elizabeth Harris 19 AUG 1793 All Saints, Wakefield, Yorks
Agnes Meansforth baptised 14 Sep 1794 Saint Peter, Huddersfield, Yorks Fa: Edward Meansforth
Sarah Mentsforth baptised 10 Apr 1796 Kirkheaton, Yorks Fa: Edwd Mentsforth
John Mensforth baptised 08 Feb 1798 Kirkheaton, Yorks Fa: Edward Mensforth Mo: ElizthEdward Mensford b 8 Feb 1800 bap 06 Apr 1800 Saint Peter, Huddersfield, Yorks Fa: Edward Mensford Mo: Elizabeth
Ralph Martin Mensforth b 13 Dec 1801, bap 17 Jan 1802 Huddersfield, Yorks Fa: Edwd Mensforth Mo: Elizabeth Harris
Ann Mensforth b 18 Jun 1806 bap 31 Jul 1806 Huddersfield, Yorks Fa: Edward Mensforth Mo: Elizabeth Harris
Thomas Mensforth b 14 Mar 1811, bap 12 Apr 1811 Huddersfield, Yorks Fa: Edward Mensforth Mo: Elizabeth Harris
Is this the Edward Mensforth (gardener from York) that your Sir Eric Mensforth mentioned?
This Edward Mensforth appears to have died in Huddersfield, Yorkshire in 1820
Burial: Edward Mensforth 2 Mar 1820 Huddersfield, Yorkshire
Interestingly Ralph Martin Mensforth appears to have had family come out to Australia also, he married Jane Lindsay and one of their sons.....another Edward Mensforth.....came out to Australia and died in Victoria in 1871 aged 36yrs.
This family does not at first glance appear to fit with my Mensforth's (which is a shame) as my Edward Mensforth (b 11 Nov 1847 West Derby Lancashire, d 9 Aug 1930 South Australia) has a father named Edward Mensforth (b abt 1816 North Shields Northumberland, d 16 Aug 1866 Wednesbury, Staffordshire). This Edward names his father as John Mensforth (shipwright) on his marriage........but I have never been able to find him

I thought I would try your John's family further back to see if there are any possible links and this is what I came up with as a possibility:
Possible baptism:
Edward Mentsforth bap 9 Aug 1767 Bishop Middleham, Durham, England Fa: Marton Mentsforth Mo: Ann
there is a marriage for this couple:
Martin Mensforth & Ann Suddick 11 Dec 1753 Wolsingham, Durham
and other children as follows:
Ann Mensforth bap 6 Oct 1754 Wolsingham, Durham, England Fa: Martin Mensforth Mo: Ann
Ann Mentsforth bap 5 Mar 1757 Stanhope, Durham, England Fa: Martin Mentsforth
Agnes Mentsforth bap 17 Jan 1761 Stanhope, Durham, England Fa: Martin Mentsforth
Martin Mensforth bap 27 Dec 1762 Stanhope, Durham, England Fa: Martin Mensforth
Mary Mentsforth bap 7 Mar 1765 Bishop Middleham, Durham, England Fa: Martin Mentsforth Mo: Ann
John Mentsforth bap 1 Oct 1769 Bishop Middleham, Durham, England Fa: Martin Mentsforth Mo: Ann
Elizabeth Mensforth bap 20 Apr 1772 Bishop Middleham, Durham, England Fa: Martin Mensforth Mo: Ann
I think this is likely your Edward's family as the name Martin is passed down in your line...what do you think? It puts the generation before your Edward of Yorkshire as a Durham family...which is getting closer to my Northumberland John Mensforth (shipwright)....I guess John Mensforth baptised in 1769 could be my Edward's father as he would have been about 47yrs old when Edward was born in 1816.........I wish I could be certain.......perhaps more digging is required!
I will let you know if I find anything else in my travels, would love to hear your thoughts on these records.
Thanks

Jenny