Stewart Robert Hinsley wrote:

I [seem to recall] there being advice out there to buy smaller trees, on
the grounds that they establish faster and catch up with the slower to
establish larger trees. (Possibly this only applies to bare-rooted trees.)

I was shocked at the price of 20m of "instant" 1.8m high beech hedging
grown in troughs that the architect has specced (nearly £10k) compared
to the price of 150x four year old 1.2m bare-rooted beech plants (under
£400)

Can you find a trustworthy nurseryman?

If the project goes anywhere I will certainly try to, but for now
pricing from the internet will have to do (50 trees from Paramount
Plants and 5000 bulbs from Gee Tee) about 500 shrubs left to price up!