Last Night's TV - David Jason: the Show Must Go On!, Five; Mary, Queen of Shops, BBC2

I'm not famous enough to get into Private Eye, but if I was, I should feel it necessary to begin the following assessment of yesterday evening with two confessions. And since I plan to be sufficiently famous for Private Eye some day soon, I shall report both of them herewith in a shameless declaration of intent, and conscious that what follows is a prime candidate for that charming column Order of the Brown Nose.

The first is that, growing up in Tooting in the 1990s, the only one of my five heroes who was a fictional character was Derek "Del Boy" Trotter, whom you will remember from Only Fools and Horses. Being a chippy south London boy, and having constructed my personality so as to compensate in exuberance for what I lacked in height, Del Boy was recommended to me as if by celestial decree.

The second is that, in the otherwise hazy years immediately after graduating from university, I was proud to answer enquiries as to my occupation with the revelation that, for an hour and a half each morning, my ugly, corpulent visage could be espied on Five, because I was the mic boy on The Wright Stuff. I was nearly famous then, too.

Solidarity being what it is, I have long felt that Five is an under-rated broadcaster, and together with my affection for all things Del Boy this predisposed me to a favourable opinion of David Jason: the Show Must Go On!, in which that wonderful actor, whose greatest creation Del Boy was, starred. In fact, the show surpassed even my highest expectations.

Its method was exceedingly simple. Take a trusted formula – famous person visits ailing small institution, promising redemption and glory – add some human drama and narrative tension; and from the outset make tangible what Frank Kermode in his 91st year still calls "the sense of an ending". It's when endings go from imminence to immanence, you see, that plots compel attention.

The Argosy Players of Hillingdon are an amateur dramatic company. Practising in the Ruislip Conservative Association, they represent a motley cross-section of lower-middle-class and upper-working-class suburbia, representatives of the two million British people, we were told, engaged in this pastime.

Jason had three weeks in which to convert these amateurs into professionals, so that they could play to a sell-out crowd at the Playhouse in London's West End. His raw material was captivating: ambitious part-timers. There was balding Sam, a school teacher who was the innovative director; good-looking Matt, a fireman used to playing leading roles who gets cast as a donkey; and cocky Samir, who thinks if local boy Dev Patel can make it into Slumdog Millionaire, so can he.

There was "full-time housewife" Yvonne, whose mum died last year, whose husband has cancer, and who is overwhelmed with emotion when creeping the boards alongside Jason; there's Paul, with multiple sclerosis, who refers to himself as a cripple; and Angus the IT manager and Carol from an RAF base, both of whom got into amateur dramatics to escape the mediocrity of their lives.

As they munched fish and chips ahead of rehearsals, and sank London Pride after, it was impossible not to warm to these spirited players, and to feel a little reassured by the existence of their association. The customary pre-finale injection of tension – will it be all right on the night? Does the lead actor have what it takes? – was a little overdone, but you had to will their success, because of Jason.

He's an avuncular-looking bloke at the best of times, but there was never any doubt over the sincerity of his commitment to these adopted children, and his own love of the stage was conveyed through images of his debut in Friern Barnet, circa 1958. He added stardust – one little girl screamed "Lovely jubbly" as he left – and his finest achievement was not the success of the performance, or the exhilaration of the actors – Yvonne's husband made it to the show after all – but his amplification of the most pleasing message of the night: the moneyed classes do not have a monopoly on theatre. The Argosy Players commanded our affection by being that glorious thing: extraordinarily ordinary.

That description would probably be deemed an insult to Mary Portas, or rather, Mary, Queen of Shops. It's rather funny how a show with ostensibly the same format as above – famous person, turnaround tale – could be so much less appealing. Half the trick is in production, I guess; but then Portas is no Jason. "I made my name in luxury and fashion retail," she said, "but now I'm heading into a whole new world" – of bakeries. That instantly deprived her of her chief claim to authority, which is what this sort of format hinges on.

And I don't know about you, but I've had enough of intrusive, faux-saviour women telling poor people how to run their lives. They promise salvation while in fact being slavishly devoted to their own advancement. No doubt Portas is an estimable lady and a sharp business brain, but she took the worst of non-doctor Gillian McKeith and Supernanny Jo Frost, added a sharp hair cut and ineffective pieces-to-camera, and cheesed me right off. "Don't do any more filming in my bakery!" screamed her chosen victim at one point, and you realised it was she who commanded our sympathies, not her putative liberator. This time, you sensed the end couldn't come soon enough.