Hi Eric and Maree,

yes, I did see the post but I didn't want to jump in too soon in case it looked like I had the answer and stopped someone else! Isabella is one of my mysteries I'm afraid - only aware of her through the IGI so far and can't see a birth or parents for her in Cavendish or Corjeag - the likely contender turned out to have married Charles Caine in Kirk Michael.

One other Gelling/Cavendish pair I found by chance when looking through the Braddan marriage register -

Wm Gelling & Alice Cavendish both of this parish were married in this church by licence this 20th Novr 1798 by me Robt Quayle vicar.
This marriage was solemnized between us Wm Gelling, Alice Gelling late Cavendish
In the presence of Robt Gelling, Tho Curphey.

I don't know who she is either - perhaps they're sisters!