Hart's contributions to children's television include the design of the ship logo used by Blue Peter and the show's badges, and the animated character of Morph, who appeared beside him on his programmes following his introduction in the 1970s.

[3][failed verification] The outbreak of the Korean War (25 June 1950) saw him being re-commissioned in the Territorial Army, attached to the Royal Artillery, from 23 November 1948 to 1 July 1950.

As well as demonstrating small-scale projects (the type that viewers might be able to do) Hart also created large-scale artworks on the television studio floor, and even used beaches and other open spaces as 'canvases'.

One of the pieces of easy-listening vibraphone music accompanying this feature—"Left Bank Two", composed by Wayne Hill and performed by The Noveltones[10][11]—has passed into British TV theme lore.

The ink and watercolour galleon, believed to be the inspiration for the Blue Peter logo and badge, was originally drawn by Hart for "Hooray for Humpty-Dumpty" on Saturday Special, in 1952.

[6] On 28 December 2006, it was announced during the reunion programme It Started with Swap Shop that Hart was in poor health, though this was not elaborated upon until an interview with The Times published on 30 September 2008, revealing that two strokes had robbed him of the use of his hands and left him unable to draw.

[15] Hart's funeral took place in the village of Shamley Green, where he had lived for more than forty years and he was buried in the churchyard of Christ Church.

[16] On 1 March 2009 a flash mob, organised through Facebook, paid tribute to Hart with around two hundred Morph figures displayed outside the Tate Modern art gallery.

[20] In February 2015 a wave of tributes (followed by corrections) appeared on social media sites over a period of two days, when an individual mistakenly read a 2009 report of Hart's death and, missing the dateline, published it as news on Facebook, from which it was later transferred to Twitter.

The collection, which included a sketch of a galleon, created for the 1952 Humpty Dumpty story and subsequently used by Hart as the basis for the Blue Peter logo, was thought likely to sell for around £20,000.