Acording to the caption the exhibition was in Bulgaria! -see quote
Museum visitors pass a scale model of a tank designed by Italian polymath Leonardo da Vinci. The tank, designed more than 500 years ago, would have been powered by men and horses. It is one of 31 scale models of his inventions currently on display at the Sofia City Art Gallery, Bulgaria.
That model, or one very like it, was in the foyer of the V&A when I went before Christmas. There was an insane wait for entry to the Leonardo exhibition (timed tickets) and anyway I wanted to see the medieval house exhibition.
Leonardo Da Vinci: Experiment, Experience, Design V&A
Leonardo da Vinci: Experience, Experiment and Design explores the most fundamental aspect of Leonardo's work - how he thought on paper.
In addition to the drawings, the V&A has commissioned nine animations from Da Vinci's written plans from Cosgrove Hall and created some models from his drawn plans - including a miniature tank, a parachute and a hand glider, for this exhibition looking into the mind of Leonardo Da Vinci.
THE EXHIBITION RUNS 14 SEPTEMBER, 2006 - 7 JANUARY, 2007 AT THE V AND A, LONDON
It would seem that there must br two such models, one commissioned by the V&A and one part of the permenant exibition in Sofia - if the BBC captio I quoted is to be believed.
The Sofia exhibition is not permanent - it started on 12 January 2007 (http://tinyurl.com/2x3r79). It seems too much of a coincidence that two unrelated exhibitions would be on one after the other... Presumably, it is a single travelling exhibition.
Can I recommend two historical novels The Medici Guns and The Medici Hawks both featuring young Len putting his inventions to practical use. Authors Martin Woodhouse and Robert Ross. I believe that there is a film of the former but I have not seen it but if its like many Hwood book adaptations I'd stick to the book.