Hi kilmartin,
Thomas Martyn (a yeoman) died when Jane was only a child 1780 so his will will not help with Trewartha but will certainly list his holdings.
http://crocat.cornwall.gov.uk/dserve/DServe.exe?dsqApp=Archive&dsqCmd=Index.tclI know some of the properties in the hands of members of the Martyn family in Crantock were Tregunnel (Jane's Brother's family) Penpol, Trevemper and I am sure many, many more. A grand house behind a high wall certainly sounds likely.
Joseph in 7th Hussars, I can't put my finger on but Joseph is certainly a family name. Jane's Grandfather was Joseph. Her brother William had a son Joseph King Martyn in 1807. Certainly sounds like we are onto something here, doesn't it.
Checking for William Trewartha Wills seems mainly Gwennap. Any likely baptisms on IGI were Gwennap. So we see there were William Trewartha's who were Yeomen in Gwennap previously. (The Redruth one isn't him, I wondered - 1826 but found that burial - he was 21)
Interesting as far as Burials there is a Jane buried Kenwyn 1830, the problem is her age. Lists her as 54 where she should be 60. I find these educated members of the gentry are usually very accurate with ages, just had to wonder with your William listing Kenwyn at marriage.
Now I realised I mucked up the name yesterday. With only the two baptisms listed I looked for burials and I did notice that buried on the same day as Mary Ann's baptism was Mary TREWATHA (No R) I looked back at the two baptisms and there is no r in those either. (Williams a little indecisive, but Mary Ann's clearly no r) Another point of interest is baptised on the same day directly under Mary Ann's baptism I note
Mary Trewatha BRAY daughter of William and Ann Bray (Or I think that is the surname)
No idea if or what the connection may be there - and with this burial being on the same day, unfortunately nothing else is mentioned. We do often see in these pre-form burials that it will be stated with a child as son/dau of. This fellow did not do it as I see none listed as children and some of the burials during this era must have been. The baptism lists Mary Ann, the burial lists just Mary so looks like it was probably not the same person. An earlier daughter named Mary is a possibility I guess (or some other connection) Just thought it was worth mentioning.
I think printouts of those two St Clement marriages should be the next step here. Do you know what the Hitchins family did?
Kris