In the famous Nicolas Roeg film Don't Look Now, Donald Sutherland keeps catching sight of a small figure in a red coat and hood, flitting through the streets of Venice. One moment this vision is gliding over a humpbacked bridge, the next it is in the corner of a small piazza. He believes that it is the unquiet spirit of his dead daughter, but unfortunately it turns out to be a psychopathic dwarf with a nifty line in jugular- cleaving.

That is why the opening titles and some of the sequences in Sister Wendy's Story of Painting (BBC1, Sunday) unsettle me, for - clad in the uniform of her calling - she does an awful lot of flitting and turning-up too. There she is crossing a canal, and there she is again in front of a palace, always passing through the frame of the camera - usually from left to right. If you were to step into shot and take her by the shoulder, would she turn and slice you from ear to ear with a kitchen blade?

No, for Sister Wendy is, in the best possible sense, a genuinely unquiet spirit. She cannot keep quiet. Her weapon is a boundless enthusiasm, and its somewhat exhausting quality takes an awful long time to kill its victims. It is only when you come to the end of a Sister Wendy show that you realise how tiring it has all been.

As a presenter, the ageing nun is quite brilliant. She belongs to that small group of people (of whom TV producers dream at night) whom the camera loves and who somehow love it. She comes over (as indeed she probably is) as a completely authentic lover of her subject - someone who has a deep desire to communicate her feelings.

Last week's programme was a skitter around the "Golden Age" of Dutch, Spanish and French painting; so Sister Wendy was to be discovered in front of canvases by Vermeer (a breathy Attenboroughian whisper), Rembrandt (rendered as Wembwandt by the nun's slight impediment), Velzquez and someone she called Pousson, but who I took to be Poussin. The script was succinct and immediate, if not always easy to deliver ("Wembwandt wemains Wembwandt"), and she passed through frames in Madrid and Amsterdam. But it was the pieces to camera, PTCs in telly jargon, that were phenomenal. Imagine standing alone and delivering a word-perfect speech in utterly conversational style to a non-existent audience without any significant hesitation, umming, erring, or repetition. Very hard to do. Yet time after time this is what Sister Wendy achieves. She seems to see the viewer in the lens, and speaks directly to him or her. This is a rare quality; many presenters do not possess it.

What Sister Wendy isn't, is grand. Possessing more overbite than bite, she is no historian or great contextualiser of art - no Lord Clark of Civilisation. When she looks at Velzquez's Las Meninas, set in the court of Philip IV of Spain, she describes it as the masterwork of "the best painter the world has ever seen", painted in Spain's "greatest moment". In reality, Velzquez's picture was completed nearly a century after Spain's zenith, and at a time of desperate decline, debt and huge dynastic problems. The psychological setting is nearly the exact opposite of the era of confidence that she perceives.

But I don't think that matters too much. This is a programme for a mass audience on a popular channel, dealing with what many see as "high" or inaccessible art. It is Sister Wendy's love for art, not her scholarship, that many will be attracted by - the rest can be got from books anon. For the time being I am content to discover how she handles this afternoon's subject, "The Age of Wevolution".

The same passion for his subject, if slightly more restrained, colours historian Richard Holmes's rambles over old killing-fields in War Walks (BBC2, Friday). Grey-haired, moustached and bespectacled - a kind of martial Brother Wendy - the professor is most often to be encountered on long strips of road in the flatlands of Flanders and Picardie, telling us about the horrors and pleasures of war. Unlike our Wend, Richard comes across personifications of the past to help him illustrate what battles were really like; a knight here ("that looks heavy"), a cuirassier there ("mmm, that looks heavy"), a musketeer ("gosh, that looks heavy") somewhere else.

And he meets loads of farmers. That's because they live on battlefields, often in houses used for first aid or as strongpoints, and plough meadows which still uncover bombs, bullets and bones. For some reason these farmers never seem surprised to open their doors and discover a mildly eccentric British historian, camera operator, soundperson, producer, production assistant, driver, interpreter and liaison officer for the Belgian National Trust on their doorsteps. Instead they immediately invite everybody down to the cellar to inspect the old graffiti. I suppose that those who live in the cockpit of Europe are used to such large-scale invasions by now.

Brother Richard does two other things that Sister Wendy doesn't. He specialises in moving PTCs, speaking as he walks along, or sliding down the side of an old shell crater, sitting down in front of the camera, pulling out a First World War field dressing, and talking about it. This too is not easy. One day we will see the out-takes where he went arse over tit down the side of a hill, or pulled 20 Bensons out of his pocket by mistake.

And Sister Wendy doesn't handle rusted old grenades. "Are these dangerous?" he asked a farmer on the Somme last week, tossing an old bomb in the air. "Very dangerous," came the reply. "But it's all right for the professionals to handle these shells," Richard reassured anxious viewers. That's all right then; the next time there's a bomb here at Canary Wharf - and the Disposal Squad gets delayed - a phone call to London University will bring 20 professors, all clad in flat caps and padded green jerkins, to our aid. "Stand back," they will yell, "I am a historian."

Better they than Esther Rantzen, if you ask me. Give me an enthusiast over a cold-blooded professional any time. Like many readers I was a regular watcher of That's Life in its early days, when the consumer/comedy mix was still relatively fresh. Esther was one of us. Then I admired her for the way that her ChildLine crusade helped make us aware of the extent of child abuse in this country.

But something has gone badly wrong. In The Rantzen Report (BBC1, Monday), she managed to be shrill, overblown and patronising, bringing the cold passion of the crusader to bear on the harmless minutiae of people's lives. Last week's topic was "Neighbourhood Rage", and Esther's opening link, delivered with glass-shattering volume and pitch, talked of "each day a nightmare", and disputes "escalating into a vicious all-out war". In the studio she had a number of those involved in such battles, and from the start she made it clear that she thought they were all silly. One woman, who had suffered from the constant and dreadful piano-playing of the lady next door, was berated for not having complained directly to her. "You daft things!" said Esther, with gruesome familiarity. Mrs Lovet, who said she "lived for her music", was prevailed upon to demonstrate her lack of technique on an instrument in the studio. "A round of applause for Mrs Lovet!" demanded Esther.

Having given us small glimpses of the most extreme examples of bad neighbourliness - without any analysis whatsoever of its psychology or frequency - Esther then delivered herself of the platitude that "if only we could learn to live and let live", then all our troubles would be solved. Sister Wendy would never have allowed such a ghastly piety to escape from her lips. This may be because she is a nun, not a showbiz personality, and thus leads a less sequestered life.