Hindsight is a wonderful thing.
In April 2005 I decided to spend some of my mother's legacy on a solar hot water panel. At the time, full domestic PV was so expensive it was pretty well non-existent, so I didn't bother with that. The system I went for was the aesthetically very pleasing Solartwin system. This takes cold water from the bottom of the hot water tank, pumps it up to the panel and then back into the top of the tank. The warm water floats on the cold and over an hour or two the entire tank is warm and ready to "go round again". Each cycle warms the water by up to 15°C, depending upon how high in the sky the sun is, and the electric pump is driven by a small pv cell, so the pump only operates when the sun shines, and of course that's at the same time as the water warms up. Very elegant. The only problem was that we live in a hard water area and in order to prevent the panel furring up it was necessary to install a softener. Never mind, soft water is very pleasant for showers and washing machines etc. For 75% of the year a sunny day will deliver huge quantities of free hot water. It's lovely.
After about 5 years it was clear that the pump wasn't working as it should and Solartwin replaced it under guarantee. Now, a further 5 years on, the second pump has packed. The trouble is, so has Solartwin. An innovative pioneering product has been overtaken by the mass market brigade, a bit like good old Solidlights.
Solartwin's website still exists, but the guy running it no longer sells or installs panels. He just flogs pumps to people like me. He's charging £380 - yes, £380 - for a small 12v 5w brushless pump. I suspect that he's in breach of trade descriptions because the website clearly shows a load of solar panels for sale and he doesn't sell them. There's another company, Thermatwin, who are doing the installations. They charge £266.18 + VAT and carriage for the same pump.
OTOH, a quick look at Amazon and Ebay indicates that fish-tank and pond pumps, which are of the same voltage & wattage, can be picked up for about a fiver. They are all described as "brushless" pumps with a life cycle of about 30,000 hours. Even if the sun shines all day and every day, that should be good for about 5 years.
Experiment time.
PS I have learned from a 3rd party that Solartwin used this pv panel method out of sheer purism. They could have used a much cheaper mains-powered 12v pump and the electricity cost would have been about £3 a year, with a thermal controller deciding when to turn the pump on and off. We could, if necessary, retrofit this.