Complex sentence: The shingle spit that blocked the river, Orford Ness, is now several miles in length and owned by the National Trust, previously being a secret military base where tests with radar were carried out.
Split into simpler sentences: The shingle spit that blocked the river has continued to grow and is now itself several miles in length and known as Orford Ness. Before the National Trust took ownership of this land, it was the site of a secret military base where tests with radar were carried out.