  We study the release of energy during the gradual phase of a flare,
characterized by faint bursts of non-thermal hard X-ray (HXR) emission
associated with decimetric radio spikes and type III radio bursts starting at
high frequencies and extending to the heliosphere. We characterize the site of
electron acceleration in the corona and study the radial evolution of radio
source sizes in the high corona. Imaging and spectroscopy of the HXR emission
with Fermi and RHESSI provide a diagnostic of the accelerated electrons in the
corona as well as a lower limit on the height of the acceleration region. Radio
observations in the decimetric range with the ORFEES spectrograph provide radio
diagnostics close to the acceleration region. Radio spectro-imaging with LOFAR
in the meter range provide the evolution of the radio source sizes with their
distance from the Sun, in the high corona. Non-thermal HXR bursts and radio
spikes are well correlated on short timescales. The spectral index of
non-thermal HXR emitting electrons is -4 and their number is about $2\times
10^{33}$ electrons/s. The density of the acceleration region is constrained
between $1-5 \times 10^9$ cm$^{-3}$. Electrons accelerated upward rapidly
become unstable to Langmuir wave production, leading to high starting
frequencies of the type III radio bursts, and the elongation of the radio beam
at its source is between 0.5 and 11.4 Mm. The radio source sizes and their
gradient observed with LOFAR are larger than the expected size and gradient of
the size of the electron beam, assuming it follows the expansion of the
magnetic flux tubes. These observations support the idea that the fragmentation
of the radio emission into spikes is linked to the fragmentation of the
acceleration process itself. The combination of HXR and radio diagnostics in
the corona provides strong constrains on the site of electron acceleration.
