Duncan, both sessile and pedunculate oak are relatively common
constituents of old hedgerows and easily adapt the extreme trimming
applied by farmers. They produce lots of growth when hard pruned and
can remain as hedge occupants for many decades. Many ancient
hedgerows here in Devon have sessile oak that have been there for well
over a century and yet are barely 1.5m high! No need to worry about
them succeeding as fillers for your hedge.