MSK

I have a 4 x 3 m plunge pool and would appreciate any help with the easiest/cost effective way to utilise a solar heater. Costa Calida area.

Solarwhizz

You dont give the depth,, to be accurate will need the Cubic Meters of Water, For best results you will also need to ensure you put on a Pool thermal blanket, ensure you get the type with the Silvered reflective bottom for best effects.

The depending on type of pool, in ground , above ground, height above sea level, ambient temperature, exposed position to wind, daily exposure to sun and what you expect from it(IE temperature rise?)
As a general rule of thumb with glazed panels you will need a minimum of about 55% of the pool surface area in panels, if you use the plastic matting type this will wind up to about 80%. IE for your yours at 4x3=12. average depth of 1.5m = 18 m3 meters, Glazed panels 2mx1.1m you would need min 6M2 2 or of the plastic mat type I would go for at least 10M. There are plenty of sites which will give you pool heating calculators, to assist. Lots of people who have spare ground or space, literally just lay out yards of black pipe and run the water thru it a very low rate, seems to work!!