Bruce - thanks v much for all your comments and suggestions. I think I have probably covered most of them already in part anyway :

1) Ann was married in Penzance (Madron) in 1837 - I have copy of marriage certificate. Her father John is on it under fathers details, but he isnt a witness - his brother is. So John could have been unwilling to mark a mark when his brother could sign? / be away at sea? / be deceased? (I have made enquiries and there seems no hard and fast rule about what goes under fathers details)/etc. This is last mention I have of him - nothing after this at all.

2) I have tried the British Navy route - trouble is there am told theres nothing on enlisted men. There is a mention of a John Corkill in the Manx Worthies - but is it my John? Dates would fit, but just cant say.

3) Will try those sites, thanks, but I dont think I have the resources to really get stuck into that long hard slog you mention (though I keep thinking a link is so so likely, it shouldnt be so difficult to find!). Trouble is do I really have enough to even know if I have found the right one - short of a very lucky break?

I have tried going back before 1802 in Cornwall in case John wasnt born in IOM, but theres just nothing there - maybe a pointer back to the IOM? Who knows!

4) Yes, those will be my Corkells. Have been able to make quite good headway into the family in the 1800s - its just prior to that I am so infuriatingly stuck!

Thanks for your help - appreciate a fresh set of eyes on the subject. I think sometimes am so bogged down that could be missing something obvious.