Well, I played and the results are interesting...

The pool (66,000 litres approx) responded well to heating.  The configuration ended up with 4 panels (approx 8ft x 4ft each)  These gave a temperature differential of around 12 degrees from input to output.  Water throughput was around 40-50 litres a minute.  It actually meant the pool was usable by the end of March (it was 22 degrees by then).  The biggest problem was windchill and cold nights - I was amazed how much this caused the temperature to drop and regularly resulted in losing all the previous days heating, and more.  Anyway, I'm persevering and will try a cover or more drastic measures towards the end of the year...it's currently 30 degrees - just right for me :) but the days are long and hot and the nights are warm so it doesn't cool down too much.

Most recently I decided to try heating the hot water for the house - with excellent results!  We already had a hot water tank with a heat exchanger connected to an oil boiler and with its own pump.  I disconnected this circuit and 're plumbed' it to go through a solar panel (one of the 8ft x 4ft panels I used on the pool). The hot water tank has a capacity of 200L. The water is being heated to 58.9C (138F) by the panel.
The pump (just a standard domestic central heating pump) is being run by a small (38 euros from Ebay) inverter from solar batteries and panels and I built a temperature differential controller to switch the pump on and off.  The result - free hot water.

Hope this inspires others to try out similar things - save the environment and save some money at the same time!

Nigel