HI Kevin
Thanks for your reply. Yes , a crotch strap certainly makes sense to me, particularly with the inflatables. It would be great to be able to go to a pool somewhere and test a range of life jackets, just to see if the flip you onto your back, how quickly they inflate etc
After a bit more investigating, i am leaning more towards an auto inflatable, no point in wearing a manual one when you've had the boom smack your mellon! This seems to be a good option but would like some feedback if any others are using them:
https://www.smartmarine.co.nz/products/safety/lifejackets-inflatab