Hi
Straightaway with that many connections with Somerset and Dorset you are probably looking at ancestry for Samuel outside of London. London was constantly attracting people from across the county. It is rare to be able to track families back in London through the eighteenth century because of the difficulty with records but also because of the very high death rate in London. People died and London needed constant fresh blood.
With marriages you only needed three weeks residency to be of the parish. If the marriage was by licence there may be a little more information in the licence particularly if one of the parties was under age.
The IGI's coverage of both Dorset and Somerset is very poor - below 10%. Luckily for Dorset you have the online parish clerk
http://www.opcdorset.com/Somerset also had one but it is a little less developed
http://wsom-opc.org.uk/From other sources
CROSS, Osmund
Christening Date:23 May 1739 Holy Trinity Shaftesbury, Dorset
Father:Osmund CROSS
CROSS, Elizabeth
Christening Date:2 Apr 1738 Holy Trinity Shaftesbury, Dorset
Father:Osmund CROSS
Burials
St Peter and St Paul Wincanton Somerset
19th May 1779
Osmund Cross
23rd January 1800
Osmond Cross
Blessed Virgin Mary Bruton Somerset
26th January 1798
Osmond Cross 14
Bruton and Wincanton are about 5 miles apart.
There is some coverage of Wincanton parish registers on Free Reg
http://www.freereg.org.uk/search/index.htme.g.
Baptism
1st March 1784
Osmond Cross parents Robert and Mary
baptisms in Wincanton for this couple start in 1777. No Cross baptisms prior that date on FreeREG.
Bruton registers are also on FreeReg
Shaftesbury and Wincanton are about 11 miles apart (use Google maps and check registers from the places shown in the area)
There are earlier Prerogative Court of Canterbury wills for Cross in Wincanton
Will of Mary Cross, Spinster of Wincanton , Somerset 01 November 1747 PROB 11/757
Will of Samuel Cross, Turner of Wincanton, Somerset 03 August 1738 PROB 11/691
Unfortunately locally proved wills for Somerset were largely destroyed in a bombing raid in the Second World War.
Regards
Valda