A pool cover will be the most economic way to reduce the energy demand of a swimming pool, so should be fitted first. A translucent cover allows solar heat into the pool whilst also retaining more of that heat. A cover should be completely removed before using the pool, as they can be dangerous for swimmers. A good pool cover will mean that you would need fewer solar panels to heat the pool.
Solar water heating can be a very effective way of providing hot water for swimming pools, especially for those that are used mainly in the summer. You will be trying to heat up a large amount of water by just a few degrees, so you don't need super-efficient solar water heating panels, just lots of them at a reasonable cost. Heat loss within the panels is not so important so there is no need to insulate.
A surface area of unglazed collectors equivalent to between 50% and 80% of the pool surface area is needed - depending on how sheltered or exposed the site is. If you are able to cheaply install glazed collectors then of course this area could be reduced, but I'm afraid we do not have any exact figures for it. Make sure that the panels will not be shaded by buildings or trees, and tilt them at an angle of about 15 degrees for best performance. You will also need to have a drain-down facility to avoid frost damage over the winter.
The water (and chemicals) from a swimming pool will be very corrosive, so this needs to be taken into account in the design of a system. If the pool water is to be circulated directly through the panels, then they will usually be made from a high temperature plastic material that will not corrode. If the water is heated indirectly, using a heat exchanger and a closed loop within the panels, then any type of collector could be used. However, as a large area of panels is needed cost will also be a factor, so cheaper unglazed plastic collectors are commonly used.
A swimming pool solar heating system should raise the temperature of the pool by between 3 and 6 degrees during the months when it is likely to be used. Because the temperature rise is small, a high flow rate through the panels is needed - probably 0.1 litres per second per square metre - so a circulation pump will need to be sized accordingly. The small rise also means that control systems will need to be different to a domestic system, as a differential temperature control is unlikely to be sensitive enough. The exact controls will depend on the layout of the system.
There will not be financial support available for a pool system, but the Microgeneration Certification Scheme list of installers can still be used to find solar water heating installers, manyof whom could also supply a pool heating system.