Which species of trees would grow best in this situation?

Trouble with new trees is, Bertie, that they have to be watered regularly and well for their first year, preferably their first two years, so unless you have some means of providing water for them, any and all trees will be prone to dying during the first two years. Presumably the ones on the lower part which are ok have had more water because of the slope. Any of the trees you already tried would be fine in that soil, but it might have been better to plant in autumn, not spring, and ensure they had a water supply. That applies to any tree you choose now.

we planted trees similar to you and as we planted them put a length of plastic pipe in the soil at the side of the stem and then ran a long hose across the field then watered the trees through the plastic pipe moving it regularly(tedious) this was just during very dry spells, we also planted in the autumn when they were dormant and needed less water (mostly from rain) hope it helps for the future Bertie
as to which trees you can find info on the internet and some councils will provide trees as part of a planting scheme and will help with info on which trees are suitable

Thanks to both for advice. I guess watering is an absolute must, though getting around nearly 200 trees is a huge labour when the water supply is a good way away. Must get a long hose and a good pump.
I think I might try Pamq's hose suggestion and move it regularly with drip nozzles near each tree.
Otherwise, I guess it will be rain dances all next summer!

Don't you dare, Bertie - if its wet all summer next year, we'll know who to blame. Just wanted to add, plant during October, if you can, don't wait till spring.

Try planting trees in Autumn as this gives them a winter of watering to settle them in. Most shrubs and trees grow better if planted in Autumn as the roots can get on with a bit of growing while the top doesn't want to. Try more drought resistant/ Mediterranean types too like Arbutus.

We did put in an Arbutus and I'd noticed it had survived well. Only problem is that the individual plants are quite expensive, compared to hazels, chestnuts etc. I think the advice on Mediterranean species, with the need to get them established in winter, is very helpful especially if we are going to get many more summers as dry as this one (in France). Thanks to all.

Bertiefox we felled 30 trees of the willow/poplar type and we found that the water retaining properties of the area afterwards was really poor and we had to address this before we could successfully grow anything.
Can I also suggest trees its worth considering such as
Laburnham (in 6 years this is huge), Fraxinus Mountain ash whitebeam, Robinia false acasia and Leguminosae Indigofera tintoria (true Indogo) this one always regenerates no matter how many times it gets damaged (have 2 large trees which flower April to July).