You are quite correct Eric, I did mean “Isabella” Cavendish and not Elizabeth.

Re Robert Gelling bpt 1827 Douglas:
I can find a Robert Gelling born IOM Farmer in Liverpool in the 1861c aged 38, 1871c aged 49. Wife Catherine? I can’t find him in 1881c. In the 1891c, this Robert aged 66 is still in Liverpool but now widowed. I can’t find a marriage. In the 1871c there is a real puzzle with ages given i.e. there is a son-in-law William Robinson aged 57 – older than Rbt and Catherine probably due to being written down correctly by census taker. Could this be this Robert?

Re: Jno Gelling, “mason” mentioned in will.
Could this JG bpt January 24 1824 Douglas be the s/o William Gelling / Margaret Quine and the grandson of JG and EC? This is where the connection could be of ‘John the Mason’ mentioned in the 1841 will of Robert Gelling. Can’t find him in any census so don’t know about the ‘mason’ part. The only possible JG's I can find are all farmers!

I might be up a blind alley here but they are all I could find which might – only might – shed some light.