Attack on officers raises questions about separation centres at jails in England

The country's three special units at high-security prisons are in spotlight after Hashem Abedi's assault against staff

With its high, ugly, grey concrete perimeter walls HMP Frankland looks as grim from the outside as you would expect for a place nicknamed Monster Mansion".

Since it was opened in 1983 on the leafy outskirts of Durham - near a 13th-century priory used for centuries as a holiday retreat for Benedictine monks - its inmates have included Peter Sutcliffe, Harold Shipman and Charles Bronson.