Eileen Mullen sends this report from the service:

St Martin's was full - almost 600 people.  A huge turn out of not just Bush House people, but also the senior BBC management who knew him through Board of Management etc etc. and masses of his extended family and friends.
(Rev) Colin Semper - one time religious organiser at Bush  - called him an extraordinarily good man and said how much he liked to watch Austen 'think' reproducing that characteristic semi-stutter that preceeded any pronouncment. I think that it was also Colin who recalled how brilliantly he had handled The Real Lives affair - in effect supporting the staff who withdrew their labour, but ensuring that the news still went out worldwide.
Gerry Mansell gave the main address - it is hard to realise how old he is being as incisive and eloquent as ever.  PD James read from Austen's book, Attic in Greece, Mark Byford read 'Let us now praise famous men...'  and John Tusa read How Horatius kept the bridge, from The Lays of Rome - apparently standing  in for actor John Shrapnel.
The best part was that 'grandchildren' took part. Sam Bawden sang, Matthew Bawden played the Last Post and Ottilie Kark sang a verse of 'I Vow to thee my country...' I think it was also Otillie who sang with a group from the Chicken Shed Theatre Company - a group that mixes learning and physically disabled people with the able bodied.  They were great and that no doubt contributed to the generosity of the retiring collection for their company's benefit.

It was altogether a joyous if bitter sweet occasion.  Nina Bawden was using a stick to help her walk, but looked terrific. And a jazz band played as the congregation arrived and left. It was a wonderful mix of the traditional and personal that managed to reflect the man.
Others may wish to add what I have overlooked or misrepresented of the day...
It  was filmed by a Roger Graef crew who are making a film about the British Transport Police and the Potter's Bar Crash for BBC 1 that may not be aired for a considerable time - and any footage from today would only be used with the permission of the family.