just a few thoughts on the energy output you have seen today...
that's just < 1 kwH of energy you got from your panel over the total 4 hour period.
if this were a glazed panel, for 2 sq metres, say 2kw of light falling in bright sun with a finished panel running at 50% efficiency, perhaps would have expected 1kw/hr for 4 hours, so 4kwh.
an unglazed panel offers 0% efficiency when temp is 30c above ambient, so your setup was probably doing very little for the last part of your test ( agrees with your temp recordings for the last hour).
probably not bad given the small volume of water in an uninsulated tank and an (as yet) uninsulated panel. panel may not have been inclined optimally in this test position which would have contributed to lower performance as well.
if your flow rate is very slow too this will also have given rise to low efficiency, as efficiency is greatest when the panel temp is closest to ambient air temp, and a very slow flow rate will run the panel hotter than it needs to be.
The temperature of the panel, carefully estimated with my hand to be about 65-70C, would have caused a lot of heat loss. The water in the bin was only partially stratified. Nearly totally mixed towards the end of the test run.
Interesting about the double glaze unit. It is by far the heaviest part and separating it into its two panes is next on my list.
Have almost certainly decided on putting the arrangement on the flat garage roof although this will involve slightly longer plumbing. The access there would be easier.
Also now thinking of a slight modification to the controller. Towards the end of the test exercise the car battery was getting a bit low on voltage. The pump was slowing and not all the heated water was getting pumped into the accumulator.
You will remember that the length of pipe on the collecting panel is chosen so that it contains the same volume of water as contained in the indirect coil in the DHW cylinder. I will try to devise a controller which senses when the heated water has been pumped as far as the EXIT of the indirect coil in the DHW cylinder and then turn the pump off.