Photograph of a Decca magnetic tape unit model 3000, used for backing storage on LEO II. It was first installed on LEO II/5, the Service Bureau machine installed at Hartree House in London.
According to John Lewis, in his chapter 'Glyn Mills Bank Officers Payroll' in the book User Driven Innovation (ed Caminer et al, published London: McGraw-Hill, 1996), pp. 227-240:
Magnetic tape data was written (and read) in fixed-size blocks along the tape corresponding to a tube's worth of data from the main store - 32 short words or 160 4-bit characters. The tape was pre-marked into blocks spaced to minimize the time delay and wasted tape caused by the need to be able to start and stop the tape movement between each block. The gap was subsequently used to write more data. When the tape had been written to the end in the first direction the tape reversed and the data was written 'round the bend', filling in the gaps on the way back. The whole tape could therefore be used for data and a full tape did not need rewinding after use. (p. 239)