In this work we present a novel growth method to obtain high structural quality GaN films on Si(111) by low pressure Metalorganic Vapour Phase Epitaxy (LP-MOVPE). Epitaxy by Lateral Overgrowth (ELO) is achieved starting from self organised islands on a GaN template layer, obtained after a growth mode change induced by an in-situ treatment of the template under silane (SiH4). Their density and size are controlled by the silicon treatment and growth temperature. An overgrowth initiated from such islands leads to a drastic improvement of the material quality, as determined by X-ray diffraction (XRD), atomic force microscope (AFM) and photoluminescence (PL). The full width at half maximum (FWHM) on the (0002) line of GaN in rocking curve scan is as low as 525 arcsec. The dislocations density in the sample, evaluated by AFM and transmission electron microscope is in the low 10(9) cm(-2) range. The 10K PL spectra is dominated by neutral donor bound exciton at 3.453eV with a FWHM of 11meV.