Political editor Jonathan Walker watched David Cameron and Tony Blair clash in the House of Commons yesterday.

Politics can be good for you. I have seen with my own eyes the rejuvenating effects of a good verbal brawl in the House of Commons.

David Davis was in terrible shape yesterday. He sat on the front benches, face red and craggy, looking like he hadn't slept in a week.

The shadow Home Secretary is a tough guy. When he needed extra cash as a student, he got a part-time job with the SAS.

But something was ailing him as he sat down to enjoy the first Prime Minister's Questions since the summer break.

A lot has happened since Tony Blair and David Cameron last clashed in the Commons. Mr Blair has seen off a rebellion from his own side and survived a difficult party conference.

Mr Cameron, for his part, urged us all to "let sunshine win the day", in perhaps the oddest speech of the conference season. But the Tory leader was in a good mood yesterday. When Tony Blair attacked him, he teased: "I know you've only got a few more goes."

And after questions on issues ranging from prisons to the health service, he came to the punchline.

"Does he back the Chancellor as his successor. Yes or No? I mean, I do. Does he?"

This managed to be a dig at Gordon Brown and a trap for Tony Blair at the same time.

For the Conservative leader to endorse Mr Brown implies that he thinks he will be a rubbish party leader.

And Mr Blair can't directly back Mr Brown as his successor - even if he wanted to - without undermining his own position even more.

So instead he ducked the question, accusing his opponent of trying to avoid discussing serious policies.

Whenever the Prime Minister spoke, Mr Cameron leaned forward, listening intently, sincerity etched on his face.

Then he stood up, said the answer had been a load of rubbish, and moved on to a different topic.

It was very clever, but perhaps not all that illuminating. Never mind - we had a story. Blair refuses to endorse Brown.

That's a pretty good result for the Conservatives.

And they knew it. Conservative MPs cheered, great roars of laughter and enjoyment as their man stuck the boot in.

After so many years of pain, finally their side looks as if it might have some chance of winning. You can't blame them for getting excited. But what was most remarkable was the effect on David Davis.

Slowly, he perked up. There were signs of movement, as he glanced around, taking in the scene.

A smile flickered across his face, and the dead, glazed look faded from his eyes.

By the end of it all he could be seen cracking jokes and chatting jovially with Mr Cameron, who seemed unaware of the restorative effect his words were having.

This may be the answer to the difficulties facing the NHS. Just stick patients in a room with a bunch of rumbustious MPs and see if any of that enthusiasm rubs off.