There is this possibility for Benjamin in Camborne 24 Aug 1730 son of Benjamin. If it is him you will find more information here uncluding the parents marriage
I have not had any luck with Elizabeth but it appears the Illogan registers have not survived very well, nor do they appear to be widely available. You may find this of interest
I note a submitted entry on the IGI suggests she was born 1741 in Pool Illogan daughter of William Willoughby and Grace Martin - a starting point perhaps