Here we report a method for improving the magnetic field sensitivity of an ensemble of Nitrogen-Vacancy (NV) centres in 12C-enriched diamond aligned along the [111] crystal axis. The preferentially-aligned NV centres are fabricated by a Plasma Enhanced Chemical Vapour Deposition (PECVD) process and their concentration is quantitatively determined by analysing the confocal microscopy images. We further observe that annealing the samples at high temperature (1500 °C) in vacuum leads to a conversion of substitutional nitrogen into NV centres. This treatment also increases the coherence time of the NV centres electron spins up to 40 μs, which corresponds to enhancement of the sensitivity by a factor of three. However, this procedure also leads to a loss of the preferential alignment by 34%.