Ah thanks Dave. That infers a 160 N to be about correct for what I operate in. The behaviour of the operative does change dangers and I feel many years experience in this type of safe craft suggests 160 to be "enough".
I was once turned over whilst racing but the water was only 18 inches deep so swimming out from under was difficult but possible. If I had been wearing 275 auto inflatable- I think i would still be there (or in a nearby church yard.)

Nick, I don't know if this will help 








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A life jacket or buoyancy aid's ability to support a person in the water is primarily determined by its Newton (N) rating (buoyancy level), not the person's dry weight on land. A person weighing 12 stone (approximately 76 kg or 168 lbs) will weigh very little in the water, so standard adult life jackets with a sufficient Newton rating will provide more than enough buoyancy to float them.
The key factor in choosing a life jacket is the activity and the type of water you will be in, as this dictates the required Newton rating and style of the device.

Newton Ratings Explained
Life jackets and buoyancy aids (PFDs) in the UK/EU are rated in Newtons (N), following the ISO 12402 standard. The higher the number, the greater the buoyancy and the safer the device for rougher conditions. 50N Buoyancy Aid (ISO 12402-5): A buoyancy aid for competent swimmers in sheltered waters where help is close at hand. It offers great freedom of movement but will not turn an unconscious person face-up.

• 100N Life Jacket (ISO 12402-4): Suitable for use in calm, inshore, or sheltered waters. It gives a reasonable assurance of safety for swimmers and non-swimmers but may not be able to self-right an unconscious person, especially if they are wearing heavy clothing.
• 150N Life Jacket (ISO 12402-3): A general-purpose life jacket for coastal and offshore waters. It is designed to turn an unconscious person onto their back and keep their mouth and nose clear of the water surface. This is a common and appropriate choice for most adult sailing and motorboating activities.
• 275N Life Jacket (ISO 12402-2): Designed for extreme offshore conditions or when wearing heavy protective clothing that traps air and affects buoyancy. It provides the highest level of buoyancy to ensure the user is floating in the correct position.

Sizing and Fit
The most important factor is a proper fit, not the weight range specified on the label. All adult life jackets are designed to support an adult's body weight in water. Try it on: A life jacket should fit snugly.

• Check the fit: Pull up on the shoulders of the life jacket; if it moves up past your ears or chin easily, it is too big.
• Secure all straps: Ensure all zips, buckles, and especially the crotch straps (if included) are secured to prevent the jacket from riding up in the water.

For your weight of 12 st (76 kg), any standard adult life jacket rating (100N, 150N, or 275N) will provide the necessary lift, with the specific choice depending on your intended use and the water conditions you anticipate.

My WAY too old Seago 275 auto life jacket has erroneously activated whilst stored in my dry workshop.
I view this as a fortunate exercise as i can no longer ignore its obvious need for replacement. Interestingly- The buoyancy deflated to zero over a couple of days so I presume there was indeed a tiny leak in the bladder?
Question-
What are the general thoughts ref size of new one to be bought?
I reckon MY 275 is the highest buoyancy to consider, but are 160 or 190 adequate.
I believe auto operation to be generally preferred with the only down size being capsize which would make swimming out
from under the upturned craft difficult?
Can I ask others their preferences? Auto or Manual?
I do not operate with my craft via a wrist band power cut off as I believe the down sides of such out weigh the advantage.