Hi Nigel
I do not have much experience but can share with you what I have delt with in South Africa for the past 25 years. I have seen many attempts and seen many of them maybe lift a pool´s temp by 1 degree over a 2 month period. My experience from many a pool vist has been that 24 + deg cel should be your aim in water temp as that is where it is most comfy. I am awaiting info on solar panels , design´s ,pipe length and dia. These people did a study on what works and what doesn´t and is aimed at Joe soap at home. Are you going for a pipe coil or a water-fed panel or what did you have in mind??? :)