Hi, I haven't had too much luck with the Cowan side of the family, only that James' father was also James which you will already know from the marriage certificate. William Cowan's WW1 service record on Ancestry has some quite interesting information in it but after his discharge in 1919 I don't have any details about him so would be very interested to know more about who he married and children etc! My own direct family link is through William's sister Amelia, who would have been a great grandmother of mine.
The Jessiey surname on the marriage certificate is almost correct - her father I believe was actually John Charles Jesse, who lived in the Isle of Wight and probably was deceased by the time of their 1889 marriage (the last record I have of him is from 1883 when he was invalided from the Royal Navy - he was a bandsman).
I hope that is of some interest and happy to help if you have any questions.