Official announcement is here.
"Almighty and everlasting God, who alone workest great marvels, send down upon our bishops and curates, and all congregations committed to their charge, the healthful spirit of thy grace; and that they may truly please thee, pour upon them the continual dew of thy blessing. Grant this, O Lord, for the honour of our advocate and mediator, Jesus Christ. Amen."
His wikipedia page has some things of interest.
H/T to Phil Ritchie via Twitter.
Update: Dave Walker has some info from the Oxford website:
"Bishop Stephen said: “I was born and brought up in Essex, and it is still the place I think of as home. Now I have been invited to return to this large, diverse and richly varied diocese to serve as your bishop. It is an immense privilege.
“What sustains me in ministry is the joy and beauty of the Gospel. I want us to be a Church that is gospel-centred, servant-hearted and mission-focused. I am hungry for us to be a Church that connects with every person and every community.
“I am excited by the prospect of getting to know and working alongside the parishes and communities of East London and Essex that make up this great diocese. I look forward to working with new colleagues and making new friends. Building upon the work of those who have gone before us in the faith, together we can do something beautiful for God in the communities we have been called to serve.
“For me coming to Essex and East London feels like coming home. However this is not the end of the journey. We must set our sights on the glory of God and on his son Jesus Christ and on the needs of the world - this is the path we will travel together.”
The Bishop of Oxford, the Rt Revd John Pritchard, said: “Bishop Stephen has had a highly effective ministry as Bishop of Reading. He has been widely loved and admired for his bold evangelism, obvious prayerfulness and inspired communication and will be hugely missed.
“We knew we’d lose him and are very grateful for the time we have had. Chelmsford will find itself led with imagination, courage and sense of fun. We wish Stephen and his family much joy.”
The Bishop of Bradwell, the Rt Revd Dr Laurie Green, added: “We are delighted that Bishop Stephen Cottrell is to become the new Diocesan Bishop of Chelmsford. Bishop Stephen is an exceptional man, whose abiding concern is that we all catch that glimpse of the wonder of God which can change our lives.
“He is man of prayer who has a shrewd eye for the important issues of the day. His books are always challenging and delightful, and he will bring new insights about how we should respond to God’s love and justice amidst the world’s challenges. He is family man of great warmth and charm, and we look forward to learning from him and working with him here in Essex and East London.”