wrote:

Fact is tubes cost way more than flat plates, hence for the same money you
get far more flat plate than tube, and can thus achieve much better
payback with flat plate.

Hmm, not my experience. I have 2x20tube panels which cost less than I
was quoted for flat panels (£600 for the tubes, £750 for the equivalent
panels). A lot depends on where you buy the collectors. I got mine from
someone who was honest and imported them from China, I've seent he same
untis sold as "German" which seems to mean "Made in China, exported to
Germany, sold on again to the UK" the price for those units was £3000
for two arrays of 20 tubes, but on inspection, the units were identical.

For a hot water supply in a barn I used a panel I made myself for £50.
It's not as efficient as the tubes and weighs a lot more. OTOH for that
price it's possible to make a very big array of flat panels.