When Indian independence leaders, including Gandhi, were jailed in 1942, activists set up a secret radio station to carry the message of rebellion against British rule. Among the campaigners who worked at the station was Usha Mehta, who was later imprisoned for broadcasting anti-British news and playing patriotic music. Claire Bowes has been listening to archive material of Usha Mehta and speaking to her nephew, Indian film-maker Ketan Mehta.