Thanks for your input David. I copied the MI details from the IOMFHS transcriptions from the Maughold Old Yard and didn't notice the discrepancy. However I have a photo of the gravestone and it clearly shows 23 Aug 1881.
I've also taken on board the question of his mother being Jane Cowley and sent an email to Registry Office asking them to check the entry. It's too much of a coincidence that they were born on the same day, same town but with a different mother although recognising that there were others of the same family name.
I'll get back to you.
cheers
Doug