Hi Ged
I found a tree on Ancestry which has the same wife for William Cowley as the one you've found, viz Elizabeth Eleanor Moore b 1860. Looking at all the census pages, which do correspond well with birth entries for William Cowley and Elizabeth Moore's children, the most likely reason for William's age being given as 35 on the IGI marriage record (extracted from parish records, so should be correct) seems to be that it's a transcription error (25 read as 35 ?)
The tree also gives parents for Elizabeth and their census pages through from 1841. It all seems to hang together well. The parents of William haven't been added to the tree, though.
Jean C