A quick note of thanks to all those who responded. We didn't quite manage to agree terms and the boat is now off the market until the spring. Saves me having to winterise and store it. What will be will be.
Conventional exterior paints of decent quality- eg primer/undercoat gloss above the water line would be fine in my opinion. They are generally used to face the British climate - wet,sun,cold etc etc so I can so no reason they should not be ok with correct preparation. Somebody mentioned flexible parts on boats- in these cases conventionals will crack especially if using undercoat which is heavily filled to fill imperfections. Depending on the severity of movement most surface coatings would not retain integrity anyway. This post wont be liked by the 'marine quality' profiteers haha