Thanks everyone for your feedback

The trees are supercolumns from

http://www.chrisbowers.co.uk/

I dont know what the rootstock is but a friend has these trees growing
in her garden. I expect that they need to be pruned to keep them to
the right shape but they do seem to do what they say on the tin

I dont think that the trees will shade the herbs but if they do and
the herbs fail then so be it. I am more concerned that the trees would
not do well with herbs around the base as I know that it is not
recommended to grow grass close to a young tree. I am not sure how
sharp the drainage will be in the bed as I am planning to put lots of
good compost into the bed to give the tree a good start, but again, I
would rather have conditions to suit the trees than to suit the herbs
because it is a baking hot spot in summer

I shall just go for it then ...

Sally