As long as you don't have shallow dry soil that dries out too much, they should be ok. The key with Photinia is that it really doesn't like a cold and exposed position. Luckily you say that it is quite sheltered. If the trees are not to the south of the shrubs (meaning the photinia's would be on the north side), are the trees to the north, east or west? North or east would be best as this makes the Photinia's of course south or west facing which is the warmest aspect.

They will take some shade but expect them to have a more open habit than if they were in full sun or in an open space with light all around. You may find also that there will be a tendency for them to put more growth on the side away from the trees.

Once they get up to height, you can trim them which will help them thicken up a bit. It would be interesting if you could post a photo that maybe gives an indication of what they are up against....literally!

One thing they do here: the less sun they get, the less red color there is on the new growth.

Many thanks both for your answers.

The site is reasonably sheltered, and the big trees are directly above and to the north of the site. But I am concerned about them growing "open" as you say fractal. I do want them to make a screen.

Perhaps I'll chicken out and swap them for good old cherry laurel after all. It's a bit boring but I know they'll grow densely in the shade.

You could source Portuguese Laurel (Prunus lusitanica) instead. Would give a bit more variety and it would be quite happy there and grow densely.

You could ask at the place you purchased the Photinia's from if they can get those instead as you have been told the site where you were going to plant the Photinia's is not ideal.