It's interesting to look at the result of the debate. An entry poll gave:

678       for the motion
1102    against
346       undecided

Clearly, the Archbishop and Anne Widdecombe had their work cut out as they not only had to win over the "don't know"s but also had to convert some of the unbelievers if they were to win.

After the debate, this had changed to:

268       for the motion
11876    against
34       undecided

(Some must have arrived late as the totals are not the same)

The landslide swing away from the proposal was due, in part, to the lack of credible argument for the motion, particularly from the Archbishop who seemed to take it as a given that they would win and therefore made no real effort. But above all, it was the clear, factual, well argued case put forward by Stephen Fry that won the day.

Somewhere up above, McGrath posted:
……… But it's important to remember that a debate is a performance
A good singer still needs a good song.

DC