TJ - after reading your note about the Gulval baptisms I have spent a couple of hours working on this problem - Again!
I don't know that it puts us much closer to an answer but here is what I have found.
The first think I looked for was a possible marriage for James Thomas.
Based on proximity the most likely was:-
James THOMAS married Jane STONE 13th February 1737/8 at Ludgvan
Ludgvan and Gulval are adjoining Parishes and I know there was a lot of interaction so this one did look promising.
What follows is a list of children of whom most probably belong to this marriage.
Included (in red) are the baptisms of the three children at Gulval.
Elizabeth bp. 27th December 1738 Ludgvan
John bp. 26th October 1740 Ludgvan
Edmund bp. 27th December 1744 Ludgvan
Jennyfair bp. 14th December 1746 Ludgvan (private)
John bp. 22nd July 1748 GulvalMary bp. 19th February 1748/9 Ludgvan
Thomas bp. 23rd July 1749 GulvalChristian bp. 30th November 1751 Ludgvan
James bp. 5th January 1752 Gulval son of James Thomas sojournerJane bp. 2nd February 1754 Ludgvan
Margaret bp. 16th June 1754 Ludgvan
Dorcas bp. 22nd August 1756 Ludgvan
William bp. 29th May 1758 Ludgvan
There is a definite problem with this list.
If you take a close look you will see there a number of baptisms very close together - particularly in the area between 1748 and 1752.
This just happens to be when the three baptisms occurred at Gulval.
And I could also not find a burial for the first John.
This means then that the three Gulval children cannot belong to the same family.
It would therefore be safe to eliminate the 1737 marriage to Jane Stone.
Looking not much further afield and I have come across something that might be a little more promising but once again I can give no guarantees.
There are two baptisms at St Hilary that stood out when I checked the register.
William son of James and Catherine Thomas bp. 23rd June 1745
James son of James and Catherine Thomas bp. 15th June 1755
These are the only two baptisms at St Hilary for this couple and, given they are 10 years apart, it is suggestive.
I then checked for a possible marriage and found this one:-
James THOMAS married Catherine PENGELLEY 2nd March 1744/5 at BREAGE
There are a few points to be noted here.
1. Breage borders with St Hilary
2. there are no children to James and Catherine baptised at Breage
3. if earlier work on your family is correct then Thomas and Elizabeth baptised several children at St Hilary
So the family 'might' be:-
James THOMAS m. Catherine PENGELLEY 2nd March 1744/5 Breage
William bp. 23rd June 1745 St Hilary
John bp. 22nd July 1748 Gulval
Thomas bp. 23rd July 1749 Gulval
James bp. 5th January 1752 Gulval
James bp. 15th June 1755 St Hilary
I have not yet found a burial for the first James (1752) and there may well be more children baptised elsewhere.
But this is certainly a 'workable' scenario and is probably the best we have arrived at so far.
CT