There is only one marriage between a James Paris and a Frances and that is the one you mention in 1748. On the marriage entry James is shown of the parish of St. Ann, Soho (Westminster).
Surely the baptisms below must relate to them:
Frances bp.25/10/1747 Shobdon, Herefordshire (I realise this is 3 months before the marriage)
Elizabeth bp.26/4/1749 St. Ann, Soho, Westminster
Jane bp.31/10/1751 Shobdon, Herefordshire
John bp.5/2/1754 St. James, Westminster
No other Paris baptisms in Shobdon and includes the 'Jane' you seek. James' occupation would no doubt involve travelling about - the 'gentleman' he worked for could have had his family estate in Shobdon.
Annette