16 pages, 1 table, 10 figures.-- PACS nrs.: 23.40.-s; 26.20.+f; 27.20.+n.-- Printed version published Oct 3, 2005.The β-delayed triple-α particle decay of B-12 has been measured with a setup that favours coincidence detection. A broad state in C-12, previously reported around 10 MeV, has been seen and its properties determined through R-matrix analysis of the excitation spectrum. The spin and parity are 0(+). Interference between this state and the Hoyle state at 7.654 MeV has a marked influence on the spectrum. The coupling between the two states makes it difficult to determine the resonance energy.This work has been supported by the Spanish CICYT Agency under Project number FPA2002-04181-C04-02, by the EU-RI3 (Integrated Infrastructure Initiative) under Contract number 506065, and by the European Union Fifth Framework Programme "Improving Human Potential, Access to Research Infrastructure".Peer reviewed