Nigel if you're not planning to get drenched ie surf launch then you might want to look at layering up with thermals. At the moment I've settled on a combo that should help keep me warm if I do go turtle and that's: 1mm neoprene vest, polypropylene long sleeve thermal top, rashie, water/windproof kokotat kayaking top with the wrist and neck rubber closures as the outermost layeer. Knee high sealskin socks, sharkskin pants and neoprene booties on the bottom. Beanie and brimmed legionnaire-style high vis hat on my head. Fingerless gloves on the hands. If it's raining a pair of waterproof overpants. If it's raining at night a light hooded musto sailing jacket instead of the kokotat top. Others swear by the kokotat dry pants with the integral socks and I think I might try those this season once the water temp drops out there (it's still 20 degrees).
With the neoprene I find a top with any sort of sleeve is not as comfortable as the sleeveless vest while paddling, it's kind of tight under the armpit.
Layering up means you can strip some top layers off if you're overheating. I worry about being hampered if I had to swim with the overpants and musto jacket so I only use those in very calm bay waters and always wear a lifejacket. I can swim fine with the kokotat kayaking top and the neoprene boots/sharkskins. If I'm offshore and it's raining I don't use the overpants as the sharkskins seem to stay warm even though they let the water through and there's a higher chance of needing to be able to swim well than in the bay.
Another great top combination if you want to keep it simple is 2 layers of polypropylene thermals then one woollen thermal and the kokotat top over that, very toasty. Oh and I mean the thin thermals, not the thick ones, they feel too restrictive and I tend to overheat too much.