There's so many wonderful loan exhibitions in London that I'm inclined to neglect those art treasures that we can see all year round. Absolutely free.

So when I had an hour to spare at lunchtime today, I decided that it was far too long since I'd paid a visit to the Leonardo Cartoon.

No better time than the present, because when the Leonardo blockbuster opens later this year, the hordes will descend.

Oh dear, I arrived in the quiet, darkened corner where you'll find the Virgin and Child and St Anne and John the Baptist, just as a tour party filed through ... I hung back and watched them. Shuffle, pause, blink, two-three seconds average attention span, that's it, we've done Leonardo.

And then I was blissfully alone ... just me and Leonardo ... and I sat and gazed until it was time to get back to work.

When I was about 11 or 12 - before helicopter-parenting was invented - I used to gad all over London on my own. (Nothing's changed then!)

And I used to spend hours in the National Gallery. I loved the soft lines of this work, and the gentle faces. I haven't been back to see it since my own mother died, so maybe that's why I was so struck today by the tenderness of the pose - the mother perching on her own mother's knee. (Mustn't get over-sentimental. I'd have flattened my mum if we'd tried this at home!)

I did think how lucky I was to have spent so many lunchtimes here over the years.

It has made me a very fierce defender of free museums.

And the rights of 12-year-olds to grow up without too much adult interference.