I'm just going to email OSO (the dual-coil hot water tank suppliers) to ask them directly, because their installation diagram has flow below return and 'most every diagram of a solar DHW system I've found in the last two days has them the other way up.

Colour me confused...

I also happened across the attached leaflet from the EST which contains info on lots of things I wish I'd know about to ask right back at the start. It's guidance for professional installers and designers of the systems themselves.

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Simon