I think Allan Chapman would have been a good shout;
http://www.youtube.com/watch?v=Cn_L1BvUGvg
He's got the credentials, and has guested on the programme on a number of occasions. However, I think his health is in question. Anyone who has heard him recently will have noticed a deterioration in his speech:
http://www.youtube.com/watch?v=rg88XvOZOEw