Have you considered the possibility of Phytophthora? Its symptoms manifest in a similar way and can be exacerbated by irrigation (as Suze says i'd do away with that).
I wouldn't trust any soils installed by Housebuilding contractors as they more often than not ship in the cheapest as-dug material they can find. My advice would be before throwing money at a replacement get that area well and truly dug out and replaced with some washed free draining material and a BS3882 compliant topsoil.
Edit - just read your last post - all the best with your renewed attempts!
Thankyou for the update. You really do have difficult growing conditions. If you have never had leaf spots or leaf damage they must be fairly happy andyour garden does look fairly sheltered. I hope you have just been unlucky and that just that one Photinia plant struggled particularly badly due to the soil conditions. Your new plant could be in a rain shadow from the wall as mentioned You will have to make some decisions on whether or not to water over the winter months, not an easy call. Watering for two years after planting may be necessary too.
I have worked as a Gardener for 24 years. My latest garden is a new build garden on heavy clay.
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https://www.rhs.org.uk/disease/phytophthora-root-rot
I wouldn't trust any soils installed by Housebuilding contractors as they more often than not ship in the cheapest as-dug material they can find. My advice would be before throwing money at a replacement get that area well and truly dug out and replaced with some washed free draining material and a BS3882 compliant topsoil.
Edit - just read your last post - all the best with your renewed attempts!
I hope you have just been unlucky and that just that one Photinia plant struggled particularly badly due to the soil conditions.
Your new plant could be in a rain shadow from the wall as mentioned You will have to make some decisions on whether or not to water over the winter months, not an easy call. Watering for two years after planting may be necessary too.